diff --git a/include/monst.h b/include/monst.h
index a94bf2bc7..3a9bf151a 100644
--- a/include/monst.h
+++ b/include/monst.h
@@ -55,16 +55,17 @@ enum m_ap_types {
     M_AP_MONSTER   = 3  /* a monster; mostly used for cloned Wizard */
 };
 
-#define MON_FLOOR        0x00
-#define MON_OFFMAP       0x01
-#define MON_DETACH       0x02
-#define MON_MIGRATING    0x04
-#define MON_LIMBO        0x08
-#define MON_BUBBLEMOVE   0x10
-#define MON_ENDGAME_FREE 0x20
-#define MON_ENDGAME_MIGR 0x40
-#define MON_OBLITERATE   0x80
-#define MON_STILL_ARRIVING 0x100
+#define MON_FLOOR          0x0000
+#define MON_OFFMAP         0x0001
+#define MON_DETACH         0x0002
+#define MON_MIGRATING      0x0004
+#define MON_LIMBO          0x0008
+#define MON_BUBBLEMOVE     0x0010
+#define MON_ENDGAME_FREE   0x0020
+#define MON_ENDGAME_MIGR   0x0040
+#define MON_OBLITERATE     0x0080
+#define MON_STILL_ARRIVING 0x0100
+#define MON_PARKED         0x0200
 
 #define M_AP_TYPMASK  0x7
 #define M_AP_F_DKNOWN 0x8
@@ -212,6 +213,10 @@ struct monst {
 
 /* dead monsters stay on the fmon list until dmonsfree() at end of turn */
 #define DEADMONSTER(mon) ((mon)->mhp < 1)
+/* vault guards intentionally remain on the fmon list at 0,0 until
+   the temporary corridor is dealt with */
+#define PARKEDMONSTER(mon) ((mon)->isgd && (mon)->mx == 0)
+/* eventually, we'll be able to use (((mon)->mstate & MON_PARKED) != 0) */
 
 #define is_starting_pet(mon) ((mon)->m_id == svc.context.startingpet_mid)
 #define is_vampshifter(mon) \
diff --git a/src/detect.c b/src/detect.c
index 11c370a05..ef9348001 100644
--- a/src/detect.c
+++ b/src/detect.c
@@ -345,7 +345,7 @@ gold_detect(struct obj *sobj)
 
     /* look for gold carried by monsters (might be in a container) */
     for (mtmp = fmon; mtmp; mtmp = mtmp->nmon) {
-        if (DEADMONSTER(mtmp) || (mtmp->isgd && !mtmp->mx))
+        if (DEADMONSTER(mtmp) || PARKEDMONSTER(mtmp))
             continue;
         if (findgold(mtmp->minvent) || monsndx(mtmp->data) == PM_GOLD_GOLEM) {
             if (mtmp == u.usteed) {
@@ -433,7 +433,7 @@ gold_detect(struct obj *sobj)
             ugold = TRUE;
     }
     for (mtmp = fmon; mtmp; mtmp = mtmp->nmon) {
-        if (DEADMONSTER(mtmp) || (mtmp->isgd && !mtmp->mx))
+        if (DEADMONSTER(mtmp) || PARKEDMONSTER(mtmp))
             continue;
         temp = 0;
         if (findgold(mtmp->minvent) || monsndx(mtmp->data) == PM_GOLD_GOLEM) {
@@ -497,7 +497,7 @@ food_detect(struct obj *sobj)
                 ct++;
         }
     for (mtmp = fmon; mtmp && (!ct || !ctu); mtmp = mtmp->nmon) {
-        if (DEADMONSTER(mtmp) || (mtmp->isgd && !mtmp->mx))
+        if (DEADMONSTER(mtmp) || PARKEDMONSTER(mtmp))
             continue;
         for (obj = mtmp->minvent; obj; obj = obj->nobj)
             if (o_in(obj, oclass)) {
@@ -561,7 +561,7 @@ food_detect(struct obj *sobj)
                 map_object(temp, 1);
             }
         for (mtmp = fmon; mtmp; mtmp = mtmp->nmon) {
-            if (DEADMONSTER(mtmp) || (mtmp->isgd && !mtmp->mx))
+            if (DEADMONSTER(mtmp) || PARKEDMONSTER(mtmp))
                 continue;
             for (obj = mtmp->minvent; obj; obj = obj->nobj)
                 if ((temp = o_in(obj, oclass)) != 0) {
@@ -666,7 +666,7 @@ object_detect(struct obj *detector, /* object doing the detecting */
         u.usteed->mx = u.ux, u.usteed->my = u.uy;
 
     for (mtmp = fmon; mtmp; mtmp = mtmp->nmon) {
-        if (DEADMONSTER(mtmp) || (mtmp->isgd && !mtmp->mx))
+        if (DEADMONSTER(mtmp) || PARKEDMONSTER(mtmp))
             continue;
         for (obj = mtmp->minvent; obj; obj = obj->nobj) {
             if ((!class && !boulder) || o_in(obj, class)
@@ -736,7 +736,7 @@ object_detect(struct obj *detector, /* object doing the detecting */
 
     /* Objects in the monster's inventory override floor objects. */
     for (mtmp = fmon; mtmp; mtmp = mtmp->nmon) {
-        if (DEADMONSTER(mtmp) || (mtmp->isgd && !mtmp->mx))
+        if (DEADMONSTER(mtmp) || PARKEDMONSTER(mtmp))
             continue;
         for (obj = mtmp->minvent; obj; obj = obj->nobj)
             if ((!class && !boulder) || (otmp = o_in(obj, class)) != 0
@@ -807,7 +807,7 @@ monster_detect(struct obj *otmp, /* detecting object (if any) */
      * with positive hit-points to know for sure.
      */
     for (mtmp = fmon; mtmp; mtmp = mtmp->nmon) {
-        if (DEADMONSTER(mtmp) || (mtmp->isgd && !mtmp->mx))
+        if (DEADMONSTER(mtmp) || PARKEDMONSTER(mtmp))
             continue;
         ++mcnt;
         break; /* no need for full count, just 1 or more vs 0 */
@@ -826,7 +826,7 @@ monster_detect(struct obj *otmp, /* detecting object (if any) */
         cls();
         unconstrained = unconstrain_map();
         for (mtmp = fmon; mtmp; mtmp = mtmp->nmon) {
-            if (DEADMONSTER(mtmp) || (mtmp->isgd && !mtmp->mx))
+            if (DEADMONSTER(mtmp) || PARKEDMONSTER(mtmp))
                 continue;
             if (!mclass || mtmp->data->mlet == mclass
                 || (mtmp->data == &mons[PM_LONG_WORM]
@@ -969,7 +969,7 @@ display_trap_map(int cursed_src)
     (void) detect_obj_traps(svl.level.buriedobjlist, TRUE, cursed_src, NULL);
     (void) detect_obj_traps(fobj, TRUE, cursed_src, NULL);
     for (mon = fmon; mon; mon = mon->nmon) {
-        if (DEADMONSTER(mon) || (mon->isgd && !mon->mx))
+        if (DEADMONSTER(mon) || PARKEDMONSTER(mon))
             continue;
         (void) detect_obj_traps(mon->minvent, TRUE, cursed_src, NULL);
     }
@@ -1046,7 +1046,7 @@ trap_detect(
         found = TRUE;
     }
     for (mon = fmon; mon; mon = mon->nmon) {
-        if (DEADMONSTER(mon) || (mon->isgd && !mon->mx))
+        if (DEADMONSTER(mon) || PARKEDMONSTER(mon))
             continue;
         if ((tr = detect_obj_traps(mon->minvent, FALSE, 0, NULL))
             != OTRAP_NONE) {
@@ -1643,7 +1643,7 @@ findone(coordxy zx, coordxy zy, genericptr_t whatfound)
     struct monst *mtmp = m_at(zx, zy);
     struct found_things *found_p = (struct found_things *) whatfound;
 
-    if (mtmp && (DEADMONSTER(mtmp) || (mtmp->isgd && !mtmp->mx)))
+    if (mtmp && (DEADMONSTER(mtmp) || PARKEDMONSTER(mtmp)))
         mtmp = (struct monst *) NULL;
     found_p->ft_cc.x = zx; /* needed by detect_obj_traps() */
     found_p->ft_cc.y = zy;
diff --git a/src/display.c b/src/display.c
index ffb9b2cbc..76daaa30b 100644
--- a/src/display.c
+++ b/src/display.c
@@ -1544,7 +1544,7 @@ see_monsters(void)
 
     /* loop through level.monsters (aka fmon) */
     for (mon = fmon; mon; mon = mon->nmon) {
-        if (DEADMONSTER(mon))
+        if (DEADMONSTER(mon) || PARKEDMONSTER(mon))
             continue;
         if ((mon->mstate & MON_STILL_ARRIVING) != 0)
             continue;
diff --git a/src/light.c b/src/light.c
index fd41acd73..48f3671b1 100644
--- a/src/light.c
+++ b/src/light.c
@@ -303,7 +303,7 @@ show_transient_light(struct obj *obj, coordxy x, coordxy y)
 
     radius_squared = ls->range * ls->range;
     for (mon = fmon; mon; mon = mon->nmon) {
-        if (DEADMONSTER(mon) || (mon->isgd && !mon->mx))
+        if (DEADMONSTER(mon) || PARKEDMONSTER(mon))
             continue;
         /* light range is the radius of a circle and we're limiting
            canseemon() to a square enclosing that circle, but setting
@@ -343,7 +343,7 @@ transient_light_cleanup(void)
        so need to be replaced by "remembered, unseen monster" glyph */
     mtempcount = 0;
     for (mon = fmon; mon; mon = mon->nmon) {
-        if (DEADMONSTER(mon))
+        if (DEADMONSTER(mon) || PARKEDMONSTER(mon))
             continue;
         if (mon->mtemplit) {
             mon->mtemplit = 0;
diff --git a/src/minion.c b/src/minion.c
index b161f928d..03a782727 100644
--- a/src/minion.c
+++ b/src/minion.c
@@ -43,9 +43,7 @@ monster_census(boolean spotted) /* seen|sensed vs all */
     int count = 0;
 
     for (mtmp = fmon; mtmp; mtmp = mtmp->nmon) {
-        if (DEADMONSTER(mtmp))
-            continue;
-        if (mtmp->isgd && mtmp->mx == 0)
+        if (DEADMONSTER(mtmp) || PARKEDMONSTER(mtmp))
             continue;
         if (spotted && !canspotmon(mtmp))
             continue;
diff --git a/src/mon.c b/src/mon.c
index a69a1aedf..3e86cb730 100644
--- a/src/mon.c
+++ b/src/mon.c
@@ -1230,7 +1230,7 @@ movemon_singlemon(struct monst *mtmp)
        off the map too; gd_move() decides whether the temporary
        corridor can be removed and guard discarded (via clearing
        mon->isgd flag so that dmonsfree() will get rid of mon) */
-    if (mtmp->isgd && !mtmp->mx && !(mtmp->mstate & MON_MIGRATING)) {
+    if (PARKEDMONSTER(mtmp) && !(mtmp->mstate & MON_MIGRATING)) {
         /* parked at <0,0>; eventually isgd should get set to false */
         if (svm.moves > mtmp->mlstmv) {
             (void) gd_move(mtmp);
diff --git a/src/monmove.c b/src/monmove.c
index 340c2f628..10a97675a 100644
--- a/src/monmove.c
+++ b/src/monmove.c
@@ -922,7 +922,7 @@ dochug(struct monst *mtmp)
             /*FALLTHRU*/
         case MMOVE_NOTHING: /* no movement, but it can still attack you */
         case MMOVE_DONE: /* absolutely no movement */
-            /* vault guard might have vanished */
+            /* vault guard might have vanished; PARKEDMONSTER(mtmp) */
             if (mtmp->isgd && (DEADMONSTER(mtmp) || mtmp->mx == 0))
                 return 1; /* behave as if it died */
             /* During hallucination, monster appearance should
diff --git a/src/restore.c b/src/restore.c
index 0cc82f30f..2c9a01a75 100644
--- a/src/restore.c
+++ b/src/restore.c
@@ -448,6 +448,13 @@ restmonchn(NHFILE *nhfp)
             restshk(mtmp, ghostly);
         if (mtmp->ispriest)
             restpriest(mtmp, ghostly);
+        if (mtmp->isgd) {
+            /* fixup for new bit MON_PARKED added post 5.0.0 */
+            if (!mtmp->mx
+                && (mtmp->mstate & MON_PARKED) == 0L
+                && (mtmp->mstate & MON_MIGRATING) == 0L)
+                mtmp->mstate |= MON_PARKED;
+        }
 
         if (!ghostly) {
             if (mtmp->m_id == svc.context.polearm.m_id)
diff --git a/src/steed.c b/src/steed.c
index a9c1ad7d0..536feec6f 100644
--- a/src/steed.c
+++ b/src/steed.c
@@ -903,8 +903,10 @@ place_monster(struct monst *mon, coordxy x, coordxy y)
 
     buf[0] = '\0';
     /* normal map bounds are <1..COLNO-1,0..ROWNO-1> but sometimes
-       vault guards (either living or dead) are parked at <0,0> */
-    if (!isok(x, y) && (x != 0 || y != 0 || !mon->isgd)) {
+       vault guards (either living or dead) are parked at <0,0>;
+       their mstate should have the MON_PARKED bit set (post-5.0.0) */
+    if (!isok(x, y)
+        && !(((mon->mstate & MON_PARKED) != 0) || PARKEDMONSTER(mon))) {
         describe_level(buf, 0);
         impossible("trying to place %s at <%d,%d> mstate:%lx on %s",
                    minimal_monnam(mon, TRUE), x, y, mon->mstate, buf);
@@ -912,7 +914,7 @@ place_monster(struct monst *mon, coordxy x, coordxy y)
     }
     if ((mon == u.usteed && !gi.in_steed_dismounting)
         /* special case is for convoluted vault guard handling */
-        || (DEADMONSTER(mon) && !(mon->isgd && x == 0 && y == 0))) {
+        || (DEADMONSTER(mon) && !PARKEDMONSTER(mon))) {
         describe_level(buf, 0);
         impossible("placing %s onto map, mstate:%lx, on %s?",
                    (mon == u.usteed) ? "steed" : "defunct monster",
@@ -928,7 +930,17 @@ place_monster(struct monst *mon, coordxy x, coordxy y)
     }
     mon->mx = x, mon->my = y;
     svl.level.monsters[x][y] = mon;
-    mon->mstate = MON_FLOOR;
+    /* even though MON_FLOOR is not actually a bit currently
+       (MON_FLOOR == 0) we want to preserve some of the other
+       bits that may be set. We'll probably make MON_FLOOR an
+       actual bit one day */
+
+    mon->mstate &= ~(MON_OFFMAP | MON_DETACH | MON_LIMBO | MON_MIGRATING);
+
+    /* We don't mess with these bits above:
+       MON_BUBBLEMOVE | MON_ENDGAME_FREE | MON_ENDGAME_MIGR
+       | MON_OBLITERATE | MON_STILL_ARRIVING | MON_PARKED
+     */
 }
 
 /*steed.c*/
diff --git a/src/vault.c b/src/vault.c
index 32d9e84b0..94f50d909 100644
--- a/src/vault.c
+++ b/src/vault.c
@@ -146,6 +146,7 @@ restfakecorr(struct monst *grd)
     /* it seems you left the corridor - let the guard disappear */
     if (clear_fcorr(grd, FALSE)) {
         grd->isgd = 0; /* dmonsfree() should delete this mon */
+        grd->mstate &= ~MON_PARKED;
         mongone(grd);
     }
 }
@@ -162,8 +163,10 @@ parkguard(struct monst *grd)
         remove_monster(grd->mx, grd->my);
         newsym(grd->mx, grd->my);
     }
-    if (m_at(0, 0) != grd)
+    if (m_at(0, 0) != grd) {
+        grd->mstate |= MON_PARKED;
         place_monster(grd, 0, 0);
+    }
     /* [grd->mx,my just got set to 0,0 by place_monster(), so this
        just sets EGD(grd)->ogx,ogy to 0,0 too; is that what we want?] */
     EGD(grd)->ogx = grd->mx;
diff --git a/src/wizcmds.c b/src/wizcmds.c
index 60b991db9..e7fda43ea 100644
--- a/src/wizcmds.c
+++ b/src/wizcmds.c
@@ -98,6 +98,7 @@ makemap_unmakemon(struct monst *mtmp, boolean migratory)
            mongone() -> m_detach() -> mon_leaving_level() copes with that */
         mtmp->mstate |= MON_OFFMAP;
         mtmp->mstate &= ~(MON_MIGRATING | MON_LIMBO | MON_ENDGAME_MIGR);
+        /* FIXME: will post-5.0.0 MON_PARKED need to be dealt with here? */
         mtmp->nmon = fmon;
         fmon = mtmp;
     }