Apologies for this sounding stupid, but is your clutch fully down? Its just that it sounds familiar to a situation I have been in a few times where like you, felt like the handbrake disengaged on its own without me doing anything. However, came to realize that I have ever so slightly raised the clutch unknowingly whilst doing something else i.e. putting seat belt on and hence, the hand brake disengages as it thinks you are trying to move off. I am talking a very small movement, not enough to move the car.