Key Lock

Stage 1: Ferry Terminal, Harbor Small rooms.
Stage 2: Airfield, Train Yard, Water Treatment, Sewer Branch, Radtown and Power Plant rooms.
Stage 3: Launch Site, Military Tunnel and Missile Silo rooms.
Stage 3 to 4: The Airfield airdrop and Chinook call terminals.

Key locks are used to lock doors, hatches, storage boxes and tool cupboards.