I have read several threads recently about cars rolling away with the handbrake apparently on (uk.legal). This is caused by the brake disc cooling and releasing the brakes. I was a bit sceptical about this, but I have just witnessed it !! While I watched, a year old avensis just rolled away and into the back of a mondeo. The way it moved away on the slight slope gave the impression that the brake had been suddenly and completely released, but the handle was still up. The car had been parked for about half an hour or so.
So, the moral of the story is always leave your car in gear as well as apply the handbrake while the footbrake is still applied (allows more pressure on the pads, according to manufacturer advice and logic)
mrcheerful