I was trying to park on a little steep driveway. Somehow I got out of the car when it is D gear. In seconds I realized the car is moving forward and jumped right back in the car, hit the brakes and switch D to P. The car didn't stopped. I remember buckled up sign on display and I did. Then I pushed the start button tried to stop and start it again, the car didn't stopped. The car continued to move down from driveway to backyard down the slope hitting and badly damaging a tramboline and a chair on its way and only stopped after hitting a tree. Then the engine worked when I pushed start button.I was able to reverse back to driveway again. I was lucky the kids were at school and there was a tree stopping the car to fall the little creek at the end of property line. Did I forget the car in auto shut off or simply shut the engine while it is on D? I am not sure exactly when I was able to switch gear P, it was before or after the buckle up sign. I don't remember any other warnings like "switch D to P to Park" before I left the car.(Normally it says) This acccident happened the day after we took it to service for 10k milage maintenance , oil change ,tire rotation, 27. Inspection. I am suspecting a main system malfunction because, after it came back from the 10k maintenance service, it was giving a strange alarm sound at random times when driver door opens. Alarm was louder and faster then normal warnings like remote left in the car etc. and there wasn't any warning