Its not an atmospheric and I can't believe that it's flash or flare and if its digital then I haven't a clue. If it was a transparency then I would say that it was a pinprick hole on the film.
If it was on a negative or slide I'd say it was a chemical Burn or a piece of hot ash from some idiot smoking in my darkroom!
But its digital and I cannot figure it.
it appears to be infront of the RAS chap at the tripod but hasn't burned though to a white background.