I've learn this the hard way as well. Samething happended to me couple years ago, it was around chrismas 2003. But, I wasn't as lucky as you are. Mine rolled down the steep lot around 15 feet and rested beneath the Pathfinder front end which almost came up pass the H emblem... There were no hero around that day...

What happened was, the weather was so cold, and the disks cool down and shrink a bit more than what calipers was holding on to, even it was driven for less than 5min. prior to that!

After the insident, I alway put it in gear in adition to the hand break, 1st if up hill, R if down hill. Transmision is far stronger than the breaks. Glad your car is safed!