If what you say is true, then she lied to you when you asked if anyone was with her. She said no until you brought up the car in the driveway. Sounds to me (call it a gut feeling) that this was a planned event in advance. Ever hear the expression "if you love someone, then let them go...if they don't come back then they were never truely yours"?
I feel sorry for you, it must be emotionally painful.