Not to worry! The HCHII has an anti-rollback feature. When it's in "autostop" (engine off) and you're on an incline, the car will stay put even if you take your foot off the brake. Only when you touch the gas and the engine restarts will the brake release. It works quite well!

The thread has comments (including my own!) which indicate that on rare occasions the car will roll back. Most have indicated this happens when the brakes were only lightly applied and / or if you're on a very slight incline.