Probably a key for a door in the hospital. It's covered in rust.
A small rusted key with a black tag attached.
This key used within the "Flashback" scenario. When playing in Hard and Very Hard modes, the switch on the wall inside the administrator's office is shielded with a plastic cover. You need to use this key to remove the cover so you can access the switch in order to reveal the hidden passageway leading to the auxiliary building 3F.