This was a reported problem on some Buick Enclaves. The most reliable conclusions were: something left plugged in...cell phone charger, dvd player, portable navigation unit, etc.; backed up and parked too close to a wall...for some reason the back-up sensors are still activated and can drain the battery; lights left on...interior or exterior. Take a look at Enclaveforum.net for some possible answers. Hope this helps. Oh, of course, have the battery checked...it could be old or just not holding a charge. Same goes for the alternator, or a slipping drive belt.