In the main building, there’s a secret room made up of container vans underneath it with an iron door. To open it, you have to power up three generators in three different locations.
You can fix the wiring of the generator near the bridge, south-southwest of the bog by spending 6 coppers or rewiring it if you have at least 5 INT. After fixing it, activate the breaker to fire up the generator.
Another generator is found due north of the Bog’s map marker. It has four silos and has the number “2” etched on it. The circuit board here requires 4 Circuitry or 6 Perception. After fixing it, fire up the generator by pulling up the breaker.
The final generator (marked #3) is located due west of the Bog map marker, near the edge of the island. Examine the broken pipe and either spend 5 Steel or repair it if your character has 7 STR.
After repairing all pipes, head back to the secret door and you’ll see all the lights beside the door is lit. Ready your weapon and prepare to take out the enemies inside. After taking them out, loot the containers, including the footlocker that contains the supply shed key. Head to the SE of the island to find this location, at the shore.