Sometimes Loot-Items spawn underneath the Floor of a Building, under broken Tanks or Cars, on the Top of Cabinets or Lockers, inside Walls or even floating in the Air. When you want to take them, you have to put away any Weapon, sometimes having to remove the number of Items the desired Item would occupy the Space from in the Inventory in the fitting Shape from a Backpack, to get the Option to TAKE the Loot.