This briefcase can be found behind the waterfall in Goka Falls. To get to it, jump onto the rocks on the northern side of the stream and move across the fallen tree (see small image in screenshot). Once on the southern side you can find the briefcase sitting on a crate behind the waterfall.
If you move to the eastern side of the Fort itself, outside the wall, you'll notice some huge slanting rocks (with a path beneath them) - refer to the smaller image in the screenshot. Jump across these to gain access to the other side; you may have to sprint to make it. Once across, move south and you should soon notice a small shack with a briefcase inside.
West of the Bus Stop you'll come upon a checkpoint; a briefcase is located in the hills north-north-west of this checkpoint. To reach the briefcase, approach the area from the south-east; you should be able to find a grassy slope going north (see smaller image in screenshot - it's just north of the checkpoint). Shortly after you'll notice a path leading west - follow it and you'll reach the car.