bright flashes of light every 1 to 2 minutes, no sounds, no clouds in sky

Approximately 11:30 pm to midnight, Thursday July 3rd driving westbound on Sir Francis Drake Blvd. in Fairfax California, we saw bright flash of light up entire sky.  Continued driving west and approximately one minute later saw another bright flash light up entire sky.  Continued driving up White’s Hill and into Woodacre, San Geronimo Valley approximately two minutes later saw another bright flash.   Parked car and witnessed 5 more flashes during the next 10 minutes.   Flashes may have continued but at midnight we went home.  There were no clouds in sky.  No sounds were heard that would indicate lightening.  There were no fingerlike arcs seen that would indicate lightening.   Today we leaned that two teenagers in nearby town of San Anselmo told their parents they witnessed similar flashes in sky around 10:30 p.m.  also on July 3rd.   Tomorrow I will call the Mt. Barnaby lookout station to see if the volunteer fire spotter witnessed the phenomenon.