At the center of Mosate Selao you can find a large, pale building with a balcony. A wooden box with a briefcase can be found up there. To reach it, locate the blue building to the south-east with a ladder going up to the roof (see smaller image in screenshot). Climb it and drop down on the straw-covered shelter next to the yellow building. From there, jump onto the balcony and grab the loot.
East, across the river, of the checkpoint in the north-western part of the region you can find a briefcase sitting on a rocky ledge. You can spot the briefcase from the other side of the river (same as the checkpoint). I haven't found an easy way to reach it - however, you can use the Hang glider found at the safehouse near the checkpoint; the ride can be tricky, especially since enemies at the checkpoint have a nasty habit of respawning. They only have to spot you and shoot you once to make the Hang glider spin out of control. It can be done, but it may take a couple of tries. You can also try this at night which makes you harder to spot.
East of the hut at Sehlakalase you'll find a small path going directly east (not down towards the water); the path ends in some flat rocky surface; following the right side of the rocks you can find a small ledge (see smaller image) going around to the south. Follow it and you'll find a briefcase.
On the southern side of the large island, directly south of Mosate Selao, you'll notice a small hole (or cave) blocked by a wooden "mesh". Go a bit west and you'll see a hole in the rocks leading to the water; jump in and swim inside the cave to find a briefcase.