This seems to have happened several times now. If I use my computer for a while (a few hours) and then shut it down, around ten seconds later I see this bright (as in, white light) flash, although I'm not sure where it is originating from (I'm usually halfway out the door by then). All devices still work afterwards.
My theory at the moment is that the scanner (CanoScan D646U) is the culprit. The thing is, it is usually unplugged from the mains, but is connected to the computer via USB. It cannot be powered by USB. I do think that it is somehow picking up residual current from the link and storing it, then when the power is cut it discharges.
Is there a way that I can test for dangerous USB devices?