This key can be found in a dead body in a cave behind Mad Mage Quest room. There's a rope spot west from chests where you get the rewards. Go up, then south. Go down to a room with three Cyclops and down again to a small cave with some Rats. The corpse is located in the south-east corner under some rubbish. This key opens the door to a room, over the Minotaur Outcasts' camp, where Cyclops guard Key 3610 and some other treasures. There's a Lever that allows access to the same room by moving part of the wall but it also closes the way back and can only be used once every Floor Reset.
Key 3610 is needed for the Devil Helmet Quest which is a level 30 quest but you need to get past level 40 Gate of Expertise to get key 3667.