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.
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.
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.