I was backing into my garage and the rear collision sensor beeped. I opened the door and took one foot out of car and since the driveway is up a hill, the car started rolling forward. I must of taken pressure off the brake. I tried to stop the car but was unable. The forward collision sensor did go off and my car hit my back neighbor fence and gas supply line. Thank goodness there was no one behind the fence. No one has confirmed or inspected the problem. A warning of “ car is not in park” went on when I opened the door while backing into garage.