From a bug report sent directly to devteam 16-Jan-2015 (subject:
"NH343 (NAO version) - display bug"), if the hero was blind and
levitating and searched next to 'I' (text) or '?' (tiles) which
was displayed on top of a known pool and the remembered unseen
monster was no longer there, the 'I'/'?' was removed but the spot
was redrawn as floor rather than water.