Seems to be a common problem.
Even leaving the non comfort access key in the car would be lame as you
are depleting the main battery while the car is awake at a high rate in order
to charge the little battery in the car.
some have measured the awake draw at about .5 amps and the asleep draw at about 10ma .
You can get 5 of those battery's on ebay or amazon for about .20 cents each . It cost me 2 dollars total to get them and get them to my front door.