U TRAPPED CORRIDOR. About half way down this corridor there is a giant spring-loaded mousetrap holding a nugget of gold (5 gold crescent value) as the bait. If set off, the trap would do 1-6 points of damage, but it can be easily disabled by even a non-thief.
This is only a distraction from the real trap, however. The act of disabling the mousetrap's spring releases a wire which causes sharp wooden stakes mounted in the wall to crash together, causing 1-10 points of damage to everyone within 10' of the mousetrap, unless they make a saving throw versus petrification at -2. This second trap can be found and disabled, of course, as long as the thief's character explains that they are searching for an additional trap.