Inside the house just south of the APR headquarters in Pala you can find another briefcase sitting on a wooden crate. If you approach the house from the southern side you can spot and empty the briefcase through the window (see screenshot).
You'll find a worn shack near the water at Mike's Bar. The shack is boarded up so use your trusty machete to gain entrance where you'll find a briefcase sitting on a small wooden desk just inside the door.
Across the northern bridge out of Pala you'll notice an area filled with a couple of large rocks and trees. Towards the back of this area, close to the rock surface, you'll notice a scrapped car. Next to the car is a briefcase.
Atop a cliff, south-east of the safehouse in the northern part of the region, you can find a small wooden construction, almost resembling a shack. Inside you can find a diamond briefcase and some medical supplies. I believe the only way to reach this area is by using the Hang glider that can be found to the north of here.
Driving west out of Pala you'll notice what appears to be a row of booths. Just north of these you'll find an opening between two masses of rocks. Following that, sticking to the west side, will lead you to a diamond briefcase.