In the SOUTH EAST part of EL ESTE (olive green part of the map) there is a base called ANTI-AIRCRAFT SITE XI or VALLEY PREHISTORICO. The base is an old caveman museum where you will see a giant willy mammoth and cave man mannequins everywhere.
Now go to the north part of the base on the hill and use your camera by pressing
and you'll notice the lieutenant (marked yellow) bent over by a helicopter. To left of him will be a hole with a grapple spot. Shoot the lieutenant in the head with a silent gun, or sneak up and machete him. Grab his maintenance room key card and grapple down the hole.
Now once you reach the bottom you will see the door where you can use the key card and go into the maintenance room storage area. Once in there look left for a orangeish/red panel box. Interact with the panel box and you'll cut the wires for the security cameras and alarms and your achievement will pop!