I have been deliberately avoiding killing rats until I found the special secret door in one of the first rooms off of the entrance to the central first level of Undermountain. I'm talking about the room with Drow and Duergar, where the secret door only appears when a rat wanders close enough to the corner of the room. No, the rat didn't get killed by AOE during combat; the rat just isn't there at all, anywhere on the whole level! If I try to spawn a rat in, it only spawns a summoned rat, which will not work. Is there a simple way to "cheat" a rat in to reveal the door to the secret room?

Secret doors are revealed when one of your characters successfully completes a Spot check, which is based on the correlated Spot rank, which is listed under the second tab of the Character Sheet: Skills.

Ordinarily, yes, but that's not how this one is scripted to work. The secret door will only reveal itself when both you and the rat are in the corner where the door is located at the same time.

Okay, this is ridiculous; I was willing to write it off as a freak, random occurrence, but it happened again!

Playing HotU with a completely different character, and once again, NO RAT! As far as I can tell, neither I or the dwarves use any area effect magic, but the rat is missing. Even when I check the area beforehand, no rat! I usually sneak past this room initially, but have always found the rat when I finally do enter the room, but for some reason, the rat is completely missing from the area, even if I wait around for a while before or after.

Just finished the fight in this room and same for me. It's not the first time either. I wonder if the rat doesn't get caught up in the clerics flame strike or blade barrier.