Help: Xi rolling down the driveway

I live in Colorado and when it snows and when the car is parked in the drive way, it rolls down into the street, causing a potentially dangerous situation (manly my car could get hit). My dad had the same issue the his 335xi and when he talked to a dealership they said it was the AWD system doesn't lock when in park and said it was the lack of emergency brake that caused it to roll. I know for a fact I had my e-brake engaged when it happened to my 335xi.



Has this happened to anyone else? Do you know what would cause this? And is there a fix besides parking somewhere else when it snows?