Electrical Problem W 2009 Buick Lucerne

I have a 2009 Lucerne and every year when the weather turns cold the battery seems to die. In February 2010 the battery was replaced and in February 2011 and December 2011 they decided the battery just needed charging. When I had the problem last week, AAA couldn't even jump start it to get it on the truck. The driver had me attempt to start the car and then I couldn't even get the key out of the ignition as the ignition wasn't receiving enough juice to release it. I don't know what the AAA driver did other than changing his ground from the alternator bolt to another location (block I believe) and then the car started with ease. We took it into the Chevy garage even though I knew they wouldn't find anything (on hope that a miracle would happen and they could find the intermitting open/short) Needless to say, they didn't find anything other than the battery was low. Like I told them, something has to be draining the battery to make it low on charge. I'm getting frustrated!! And the Lucerne has less than 35,000 miles on it.