Hub's 2011 'vert was dead when we pulled it from storage this spring. A charge got it going. A week later, dead again, so off to the dealer. They didn't ID any problems and just charged the battery. Then we went on vacation and - yep, came home to dead 'vert. Back to the dealer, still no problem ID'd (and GM will not reimburse the dealer to replace a battery that checks out OK). Another charge, then a few days later, dead again! Finally, the garaged ID's a bad OnStar module as the culprit that was draining the battery. They disconnected temporarily until the part came. Once installed, odd things started happening - top wouldn't go down (then it would), trunk wouldn't release, chimes every time we started moving after a stop (like at a light). We took a long drive, and the mileage (I've heard you need to go a hundred) seemed to reset the computer and everything seems fine now. As of yesterday, anyway.
So, if you end up with dead car tomorrow, have the dealer test it, and make sure they test the OnStar module. My suspicion (in our case) is that the loss of charge caused the OnStar to fail, which in turn kept draining the battery.