seeing a monster make itself invisible

From the newsgroup:  you could get "suddenly you cannot see the <mon>"
even though it remained visible.  Cited case was for an orc who drank a
potion of invisibility while being observed by a hero wielding Sting, which
causes orcs to be displayed even when they're invisible.  But it could also
happen when non-blind telepathy or extended monster detection is in effect.

doc/fixes34.4

@@ -359,6 +359,8 @@ amnesia of object discoveries would never forget the very last one
 code controlling item drops by small monsters still used pre-3.1.0 weight
 monsters who want the Amulet won't attack the Wizard to try to get it
 opening or closing the castle drawbridge via music consumes a turn
+could get "suddenly you cannot see the <mon>" while invisible mon remained
+	displayed due to telepathy or extended detection
 
 
 Platform- and/or Interface-Specific Fixes

src/muse.c

@@ -1776,12 +1776,12 @@ skipmsg:
 		} else
 		    mquaffmsg(mtmp, otmp);
 		/* format monster's name before altering its visibility */
-		Strcpy(nambuf, See_invisible ? Monnam(mtmp) : mon_nam(mtmp));
+		Strcpy(nambuf, mon_nam(mtmp));
 		mon_set_minvis(mtmp);
 		if (vismon && mtmp->minvis) {	/* was seen, now invisible */
-		    if (See_invisible)
+		    if (canspotmon(mtmp))
 			pline("%s body takes on a %s transparency.",
-			      s_suffix(nambuf),
+			      upstart(s_suffix(nambuf)),
 			      Hallucination ? "normal" : "strange");
 		    else
 			pline("Suddenly you cannot see %s.", nambuf);