When a nymph or monkey successfully steals a worn item from hero,
first the item is unworn, side-effects of that take place (most
noticeably descending when losing levitation or flight) including
feedback about such side-effects, finally "<Mon> steals <item>" and
transfer from invent to thief's minvent. If the side-effects were
fatal (such as drowning or burning up in lava), the player wouldn't
see any explanation for why that happened.
When a thief removes a worn item, give a message to that effect:
"<Mon> takes off your <item>." That will usually be immediately
followed by "She stole <item>." When the thief isn't a nymph or if
any messages were delivered after the "takes off" one, the monster
will be described by name: "<Mon> stole <item>."