It's obviously someone a few feet from the camera tossing the hubcap into the air.
When you focus a SLR camera "to infinity", ie. the maximum settable focus for distant objects, the lens has an actual physical distance at which things will still be focused. Usually it's like 50 feet from the camera. So when the camera lens is focused to infinity, things >= 50 feet away will be in focus.
Because the trees are slightly out of focus, the lens has been set shallower than infinity, which means the UFO is closer to the camera than the physical infinity (50'). It's physically impossible for an object more distant than the trees to be in focus without the trees being in focus.
When you focus a SLR camera "to infinity", ie. the maximum settable focus for distant objects, the lens has an actual physical distance at which things will still be focused. Usually it's like 50 feet from the camera. So when the camera lens is focused to infinity, things >= 50 feet away will be in focus.
Because the trees are slightly out of focus, the lens has been set shallower than infinity, which means the UFO is closer to the camera than the physical infinity (50'). It's physically impossible for an object more distant than the trees to be in focus without the trees being in focus.