Stealth is really a mess - they should be actively searching for a source of the damage (unless they truly are mindless creatures). The way you can also phase into stealth, move, always attack with surprise, exactly know where pixel-precise vision cones are etc is silly. Creatures should be allowed a perception roll (using all their senses - and some have super-keen senses) to locate either approximately or quite accuately where you are. This really means many encounters can be cheesed, which is sad.
