Car accident

Oh dear - not my car but my neighbours. I have a really super Kia Rio car when I bought in November. I am a real fan of Kia cars and had one since 2007. I had parked my car on the driveway when I returned from a trip into Cambridge today and with the handbrake on and the car locked I went in to chat to Mike. Half an hour later and after Bill had come home, we heard an alarm and my car had slipped back down the driveway and across the road and hit PAul's car and done considerable damage. The handbrake was still on and the car was still locked - mystery. Mike looked on Google and Kia have had a number of similar accidents when the disc brakes contract after 30 mins and lose effectiveness. They recommend leaving it in gear at all times - not how I was taught many years ago. The main thing is that nobody was hurt but I am pursuing with Kia.