The elevator.
The Lighthouse Elevator Key is a written note found in Dishonored.
Transcript
Captain Pinchfield,
Remind your men that the lighthouse elevator is for use by Important Personnel only, and in the case of emergencies. This key is to stay where it's at.
If anyone in your unit has legitimate need for the elevator, talk to the on-duty officer. He has a copy of the key and can grant access if he deems it appropriate.
Those are the procedures. Thank you for following them.
Location
It can be found in the gatehouse during The Light at the End, underneath the key itself.