I logged out of the game and back in a little later when i noticed two things:
a) most of my items in the inventory had been rearranged and
b) one of the items was missing (yellow box)
i things this happened due to the rearrangement of the items. i still had the required 4 free slots for the box that disappeared but these 4 slots where not in a 2x2 form so the box didn't "fit" into my backpack anymore and got "dropped".
the box was not on the ground when i logged in.