Often when I'm adventuring I find that my NPC allies can see things through walls and even floors and celings. It's worst in caves when they all rush off out of sight in two moves. Then the combat messages start. I have no idea where these guys are, but I know exactly who is being hit by what. I often find the scene of the combat later, usually several rooms away but sometimes on other levels as well.
The worst part about it is these guys tendency to wander around after that inital combat. I've lost several adventurers when the six man team gets broken up in the rush to kill a mudman or a kobold on another level and leaves thier glorious leader behind. It's a real killer because then they tend to encounter the giant/hydra/elite archer one at a time, and die one at a time. Of course I didn't bring a six man team to clear out the local mudman nest, I brought them to help with that big nasty as a group. Armok knows that certianly can't handle things as individuals, no matter how much they like to try.
What I'd like to see implemented to help with this is a sound/listening system similar to the "glowing eyes" method of seeing things in caves. Flashing dots or x symbols of various colors could signal things beyond the limited sight range but close enough to be heard. You could even adjust the interval of the flashing to how much noise the critter made. Batmen and ratmen could be intermittant flashes, only one flash every 40 seconds or so, since they don't make much noise. Since giants aren't exactly renowned for thier stealth they could flash at the normal rate used for the status/effect symbols.
Just a suggestion. But the NPCs all rushing off at something three rooms away or on another level that I can't even find is annoying.