>> Generally the rule is that if it is a terrain that a thief can hide in then only assassins, duergar, and other thieves can see them. If it is terrain a thief is unable to hide in then only acute vision and assassin detect hidden will let you see them.

I've played a few recent rangers, and this isn't always true.

My caverndweller beastmaster can't see hidden foes in caves, and my mariner hunter couldn't see hidden foes in the water. My plainsrunner explorer, however, could seem to see any hidden chars on most wilderness terrain (although I don't think I ever tested caverns or water), although that may have changed since the explorer was a long time ago.

Granted, these hiding foes are all mobs (and not PC's, since they can't hide here) that should have probably been written to be camouflaged rather than hiding, but it doesn't make sense that there are so many things a ranger can't detect in his own home terrain.

I agree with Aerdica that if there is some specific criteria to seeing hidden foes in the different terrain, it should be documented in the helpfiles, since I don't want to have to play every ranger combination to figure it out on my own. This kind of thing is an important factor to know before you start a character.