What exactly does “parked and in gear” mean? If this is an automatic transmission, and it was in “P” then it wasn’t in gear, and should not have moved whether or not the engine was running. If this is an automatic transmission, and was in “D”, “R”, “N”, “2”, or “L” then of course the car will move if you take your foot off the brake, even if the engine is not running.
Anytime you park on an incline, you should use the parking brake, and not rely on the transmission alone to keep the car from moving.