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It's your Garmin rebooting.
(Actually happened to me on PBP just after leaving one of the controls [Loudeac I think]. Edge 705 battery was dead, USB battery pack was all but dead but had just enough power for the Edge 705 to try and turn on, flash of white from the screen and then it would turn off, 30 seconds later it would do it again. I was hunting round for the person riding with a flashing front light ready to kill them. Took me 30 mins in my fucked state to work out what it was.)
I see lightning flashes out of the corner of my eyes. I always assumed it was actually lightning but it's happened too often and on too many clear days/nights to actually be lightning. Also, no thunder.