According to a newsgroup followup, a hidden pet summoned via magic
whistle could produce the same effect as the level change case (where
sometimes the glyph for unseen monster would appear unexpectedly). I was
unable to reproduce this one, but I don't see anything in the code to deal
with the situation, so I suspect that the monster is moving immediately
and being revealed before I have a chance to notice anything odd.
I assume that other situations where hidden monsters get teleported
are being handled as attacks which expose them. At least I hope so.
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