After the adventurers killed the Shadow the Gamemaster states that Nimble finds a secret trapdoor. When the characters open the door it is a door in the wall, not a trapdoor. It has to be said, anyway, that "find secret trapdoors" is a D&D gaming term including any kind of secret passageways, be it on the floor, wall or even roof.
Obvious double when Newmoon the Elf kills his player.