I think your
problem is more likely to be the discs as opposed to the hand brake lever dropping. I have had this
problem on a few cars now (Current car which is a Touran, Impreza and a Rover 420 SLI). Each car had rear disc brakes and like many have said, as they cool, away goes the car. Drum brakes dont do this as the play on the shoe grabs the drum as it trys to roll making it brake harder.
As for leaving it in gear, I am sure you know how hard it is to bump start a car, and how if you dont move fast enough before engaging a gear it stutters to a halt, imagine putting gear in and then pushing the car, you are never going to move it even on a hill (we are not talking a cliff here).
I bet as this has never happened to you before, you havent owned many cars with rear discs. You must put handbrake on alot harder and you cna ensure its on by pushing against to test it doesn't drop (as your dealer will do)
I doubt your going to get anywhere as a warranty issue.
Hope this is helpful