i have experience of this at at an oem. Generally these days is the rear brakes are disk, and have combined handbrake/footbrake calipers. You pull the handbrake on, lock up and walk off. Some time later the disk cools down, the pressure on the pad reduces a bit, the car rolls away. Happens a lot.
Way to tell if your car has a combined caliper and not a drum in disk like a volvo etc. Is to apply the handbrake normally, and then with engine running and footbrake applied firmly, re apply the handbrake. If the lever comes up more for the same force then its a shared caliper. If you look in your handbook and it recommends applying the footbrake whilst applying the handbrake then odds are the oem has had had field issues with rollaway.
Pull the handbrake on fully, leave it in gear on a hill, and wheels in.
Drums are better as a handbrake because as the drum cools it gets smaller and the brake shoe pressure increases