WHERE IT STARTS:
The Treasure Hunt is at the FDA AIRBASE OLIMPIA, OASIS PLAINS, AGUAS LINDAS.
It's in the south western area of the Airbase's restricted zone.
THE NOTE SAYS:
"TO: ALL PERSONELL
SUBJECT: BUNKER 2 ACCESS
The doors to the bunker must remain sealed at all times. Only authorized personnel with proper FND clearance are allowed past this point.
Only authorized superiors may access the cache. Any trespassers, military or otherwise, will face immediate disciplinary action.
WHAT TO DO:
Written Guide below the Walkthrough Video
Climb onto the building's roof. There is a button that opens the bunker door, and then it closes again because of a failure.
While it opens, shoot at the Gasolina Canisters inside that will blow open the hatch on top of the bunker.
Go on top of the bunker and use the now open hatch to get inside.
Follow the corridor to the end with the locked door. Shoot the lock at the fenced off area. The keycard is hanging on the wall inside.
Inside, push the door at the floor of the desk.
Pick up the chest in the secret room.