While parked in my residential garage with the engine shut off and car door closed, the vehicle suffered an un-commanded rollaway incident due to a suspected defect in the electronic parking brake or automatic-park transmission software. The vehicle was completely stopped, the driver exited the vehicle and the driver's door was closed. Moments later, the vehicle shifted out of its parked state or failed to hold, rolling backward down the driveway and colliding with a basketball pole in the driveway, causing vehicle damage. No warning chimes or dashboard alerts were observed prior to exiting. The vehicle's electronic failsafe to automatically engage 'Park' upon the door opening failed to secure the vehicle.