These boxes are usually
scattered around the map, containing useful weapons or items. You can't put the box into your
inventory, but you can move items between the box and your inventory. Be careful if your
having a fire fight near one of these, if one contains explosives and you shoot it, the box
will blow up. The Green Beret can carry boxes, so you can move them to a safer location
before looking in them.
TIP: Useful stuff can be also found in closed areas, like houses. You might
not find boxes in there, but the items will be stored in the buildings' furniture. Check
carefully and try not to miss anything.