<...So, it's a terrestrial phenomenon...perhaps a geostationnary satellite that had its solar arrays briefly oriented in such a way as to send sunlight to the chip.

That's an interesting possibility. I have never heard of reflections from geosynchronous satellites before, but I can see where it could happen. Obviously, on a five-minute exposure, a satellite in a lower orbit would have left a "streak" on the "film."

Given the time of day and location in the sky, I'm sure a cleverer person than I could calculate all the angles involved and figure out if a reflection is feasible, or if its in the right spot for such a satellite. At the altitude of a geosynchronous satellite, the satellite is in sunlight for most of its orbit.