Contact the U.S. Agent undercover at the base, retrieve his casette tape of classified intel on the black site and escape the mission area.
You have two ways of contacting the agent; one is by finding him and two is by signaling him from the west watchtower. Or use the shortcut method by following the Infiltration Achievement/ Trophy and Classified Intel Tape method below.
Turn on the watchtower light and wait for the target and his buddy to arrive. Once you found the target, you have to knock out his partner then restrain him from behind. After learning the location of the casette tape, your mission objective will be updated. You now need to head to the central control tower marked in the map.
It will easier to reach it by heading to the administration building's northwest door. There's only one guard there so you can easily take him out. After entering the red door, continue along the path until you reach a storage shed with some supplies and a jeep. If you look to the right, you'll find a ladder leading to a platform. Use that platform to reach the central control tower. Before entering, disable the security camera then pick up the casette from the table. Quickly leave the area and head for the landing zone. Board the chopper to complete the mission.
Infiltration Trophy/ Achievement / Classified Intel tape
Wait until the truck enters the administration building. Disembark from the truck and make sure to at least tag the driver so you don't accidentally knock him out. Now go around the building and climb the ladder leading to the central watchtower. Go to the central watchtower and disable the camera. Pick up the classified intel from the table then make your way to the boiler room.
You have to restrain the bald guy and interrogate him to make him drop the casette. If you accidentally knocked him out, kick him to wake him up, restrain him again and repeat the same process. If you completed this process quickly and you haven't triggered any more alerts, you should head back to the truck and wait on the back until he leaves.
If ever the driver spots you and you're forced to tranq him, just kick him to wake him up. This will put him to a temporary alert status; stay out of sight at the back of the truck and wait for him to stand down. He'll return to the truck afterward. Wait for the truck to leave the mission area and the mission will be complete. You'll unlock the trophy in the evaluation screen.