In the north-eastern part of the region you'll come upon a worn bus likely with militia hanging around. There's a briefcase on top of the bus - use the sandbags to jump onto the makeshift shelter and onto the bus.
You can find a large ledge along the cliff, in the south-western part, which has a briefcase on it. Go further west and up the slope to reach it (see small image in screenshot).
In the field north of the Underground location you'll come upon a small makeshift shelter. At the shelter you'll find a briefcase sitting next to a television set. Not sure if the TV runs on batteries or solar power, though.
A makeshift shelter is located near the southern shore of the river near the western border. Here you'll find a crate with not only some health supplies and ammunition but also a diamond briefcase.
West of the checkpoint south of Mokuba you can find a diamond briefcase. To reach it, move to the northern or the south-western side (other sides may be too steep), and climb onto the rocks and from there onto the patch of grass. You'll find the briefcase near the cliff wall.