I haven't drained the battery, but I have forgot to put it in park a couple of times. I guess it must have shut down on its own somehow. I tell you, I love this keyless entry system. My husband hates it, esp with his car, but I really like only having to have the key in my purse, walking up to the car, and it unlocks. I just wish it would unlock both doors instead of only the driver's side!!!
There is a way to fix that. When I touch the door all 4 doors open :). I forget what it is you have to do, but you hit and hold something on the remote for a certain amount of seconds and it will switch the setting to open all doors when you touch the handle instead of just the drivers door!