This page of the Far Cry 6 guide describes a treasure hunt Cache Money available in Madrugada.
Subregion: Aguas Lindas.
District: Oasis Plains.
To start the treasure hunt, you must read the note attached to a small booth.
Your objective is to reach the Bunker number 2. To do so, you must get onto the roof - you'll find a button which opens the door for a few seconds. It's not enough time to reach the bunker, and especially not to enter it. The door is blocked with exploding barrels. Your task is very simple. You must open the door and then shoot the barrels with your gun (preferably using incendiary ammo).
The barrels will explode and open the way forward, letting you go inside. Avoid the fire and keep moving forward, you can't possibly get lost.
Eventually you'll reach an office, look underneath the desk and press yet another button. This will open the secret room with a weapon inside, collecting it completes the mission.