Whilst I think it most likely that the flash has reflected from something or other on the fence - which we can't see 'cos of the bloke's head - the other thought which crossed my mind (and this is a bit way out, but then, it is photography), is what of the possibility that a small flying insect may have been caught in the flash - too close too the lens to have been recorded, but close enough to bounce some light back? As it was a slow shutter speed with flash, this does leave a option for such an occurance. Possibly. Still think it's a metal tag reflecting.

BTW, were you using filters at all? Do you keep a skylight filter on your lens? If so, have you checked it for any signs of flaws or damage?

treaclechops x