Since we now have multi-story buildings with internal walls and doors (yay!), why not secret doors as well? They would work essentially like stone mine doors. Requiring more skill than regular doors, they would appear like a normal wall of the appropriate type, but would allow passage unless a lock is applied. Would be subject to all the normal rules for internal doors, including locks and picking said locks. Detection would be possible as per stone mine doors (i.e., right-click and Examine a bit of wall you suspect contains a secret door) What do you think?