I had a saved game still in the caverns, so I tried the secret door under the stairs. I clicked and clicked around blindly and finally, I set off a trap on the door and it opened (killing two characters). Inside was the treasure.I reloaded it to see if I could do better with perception. It took Expert perception level 9 (got there with horseshoes) to "see" the door, but I was completely unable to open it that time (I finally got bored and gave up). Perception doesn't help you see how to open the door, just that the door is there. There must be a switch, but I was unable to see it with or without the Perception working. Just got it open by chance the first time.
