ToME: the Tales of Maj'Eyal

Sometimes when I use track and try to look at monsters around on maximized map, if I hover over monster, that has stealthed, the icon on map vanishes, but it still distinguishable, If area around this monster doesn't explored yet.

However, this behavior only appears time to time, which makes it hard to reproduce. Can someone explain how it works?

I think it is related to the way you sometime catch sight of something, but can't actually see what it is.