I had it happen twice. I had a non-full ammo box, a half empty mag and 1 empty slot in inventory.

A buddy died so I emptied a mag he had on him(as in, in the inventory of his dead body)... poof ammo gone. Checked the box, checked the ground, checked the mag in my inventory, checked the empty slot in my inventory.

Later on, I tried to do the same thing with a box of ammo (for the m4 that was sitting on a shelf in vicinity) and the same thing happened... poof ammo gone. If that would have happened earlier tonight, I would have been really bummed but I just got back from NE airfield and was stocked up so it really wasn't a huge deal.