Response to Alan's previous posting:
I believe that's the extended shadow from the woman(left side of the mailbox) running towards the steps/building.
Another point being that when some videos are stabilized, even using inferior quality material, sometimes events become a little clearer.
So those that question these methods might want to take that aspect into consideration.
Added on edit: The object you point out could be part of a stationary shadow from the woman in white ahead of the running woman.