Requested by one of the beta testers 13 months ago... when a visible
monster becomes invisible and vanishes, mark its map location with
the remembered, unseen monster glyph. (When the player zaps a
monster with a wand of make invisible, that only happens if the wand
type is known. I'm not sure that's right but didn't alter it....)
The request suggested also doing it for a monster who disappears by
teleporting away, but I haven't attempted to implement that.
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