this sistuation happened to me a few months back, i was wondering why my car wouldnt car after i left it at a stand still over night.
it kept happening to me everyday so i decided to call a car electrition (£50 an hour), he check the battery and found a 1amp drain when the car is switched off. he sed.. and i quote "if you got a new battery and left your car at a stand still for a week it will b flat"
after 10mins we then found out that it led to my stereo, this is becuase when i switched off the ignition my stereo would still play.
All he did was change my earth cable from the stereo to a different location and then after that it never happened again.
Try and find a different earth and see what happens, dunno if this will help.