Pacifica Hybrid Limited found completely dead in our driveway and unresponsive to the remote. I manually unlocked the driver door, no usual dash indications, opened the hood and found the 12v battery system at 2.9v. It would not start & no responses or indications. There had been no warnings or indications of any problems prior to this. The car prevented my 6 A charger from charging the battery while it was connected to the car. I jump started the car with a known good battery, removed the jump, and allowed the engine to charge the car battery some. I decided to take no chances and replace the 12v battery. I turned the car to the OFF state via the dash push button. The car was never moved out of P park. I left it to go purchase a new 12v battery. I returned 2 hours later with a new battery to find the car in the street and having knocked over my neighbors 700-800 lb brick mail box. A police report was filed, pictures taken, and I jump started the car with the new battery. It started normally and I backed it up under its own power into my driveway where I replaced the 12v battery without incident. The car has operated normally with no warnings or indications of a problem before and since the incident. The car on its own and while OFF, unlocked the transmission park pawl allowing the car to roll 80 feet down a very mild slope and across the street to crash into my neighborâs mail box when the 12v battery failed. There was no occupant, and no key fob was present. My quick sear