Right next to the western cliff, in the western part of the region, you can find a diamond briefcase sitting on a rock. The briefcase is right behind a nearby Car.
North-east of the Bus Stop (or north-west of the safehouse) you can find a collection of tall rocks. In between these rocks you can find a briefcase along with some health supplies.
West of the safehouse in the north-eastern section of the region you'll find a couple of trashed cars and a burned out truck. In the back of the truck you can find a briefcase sitting on a crate.
On the western side of Sepoko, right next to the northern observation post, you can find a small shelter with metal sheets for a roof. On the roof you can find a briefcase. To reach it, climb the observation post and jump from there onto the shelter.
West of Sepoko - directly south of the safehouse in the north-east - you can find a large tree next to some cliffs. There's a fire here as well, making the area easy to find. Right next to the tree sits a briefcase along with some ammunition.
Just off the road, south-west of the checkpoint in the south-eastern corner of the region, you can find a briefcase sitting on a large, flat rock (above the water).
There's a small path going up to the safehouse that's exactly in the middle of the three checkpoints in this region; in the middle of this path you'll come upon a briefcase.
In the south-western part of the region, close to a checkpoint, you can find a small body of water surrounded by large rocks. On the north-western side you'll notice a fireplace; right next to it lies a briefcase.
West of the safehouse in the south-western corner of the region you'll find a small body of water. Here you can find a briefcase sitting on a crate right next to a fallen, dead tree.
Up on a cliff here sits a briefcase on a crate; there's a water bottle and some explosives nearby. You cannot reach it by foot; instead you'll have to use the Hang glider located to the south-east of this place.