The Motorcycle Key is a key found in Silent Hill and the final item needed in the Kaufmann sidequest.
The key can be seen underneath the floorboards and must be retrieved using the magnet found in the motel's office. The motorcycle the key belongs to can be found in the motel garage, and after using it, the gas tank will open, revealing a hidden vial of Aglaophotis. Completion of the Kaufmann sidequest determines whether or not a good ending is achieved at the end of the game. However, the whole quest is optional.