Another thought on the flash would be the setting sun reflecting off a shiny panel of a satellite.[/QUOTE]
Maybe it was an Iridium Flare? Did it look like this http://www.w7ftt.net/iridium1.html or this http://www.assa.org.au/observing/iridium/ ? A little more about them here http://en.wikipedia.org/wiki/Satellite_flare .
Then again, maybe Marty was working on some secret weather instrument that night