Totally guessing here, since I don't have a clue...
Looks to me (and this is just a guess) that it is a weird reflection from inside your camera... like the flash glare bounced off a reflective surface on your focusing screen while the lens was open, or off the seam of the ring that holds your UV filter together on the inside or something like that (I know hoya filters have those kinds of seams).
It would be interesting to see if you could reproduce this by shooting similar pictures.