Sorry - don't mean to hog the thread!
This third car is most likely the co-worker? Wasn't there something in a previous post about the co-worker turning up around the time the 911 caller arrived?
From what I read, I dont think the coworker stuck around. I think they saw what they saw ( or say they saw... Im still not convinced...) and left. Only a day or 2 later did they come forward. Could the 3rd car be maybe another customer who pulled in... could not get gas, and just left. Not really giving it any more thought? Or had been wondering where the clerk was, saw the 911 guy and figured he had it covered and left?