Because I have not opened a Sakhal briefcase in some time I cannot comment on whether there's open space in them for this to happen, but i know on modded servers where the contents can be different, ive noticed if I have a locked briefcase and I empty ammo out of a mag the ammo disappears, assumigly into the locked case. Tested this with admin tools and indeed ammo can go into a locked case if it has space, and your inventory is full or close to it.