When logging in after safely logging out, items 2x2 in size will sometimes disappear from your inventory after the game plays tetris your items and they happen to form a pattern that isnt 2x2 the item in question wont be able to be placed in said inventory and disappear. Happens randomly in backpacks, when the backpack is either full or close to full. I had heard that it was connected with having an excessive amount of containers, but having only one of each(3) just lost my protector case. Another contributing factor could be that i carry my 2x2 items at the bottom of my backpack.