It could have been some light inside left on. I also found out that if you leave the car in gear and shut it off, the dashboard goes blank but the 12 V battery was still powering radio etc. I would assume after the car was started it was taken for longer ride to fully recharge 12V battery. I would keep checking that battery voltage and would ask dealer to run test for short, on the next oil change, but there are many ways to run battery flat so it doesn't have to be some fault with the car itself.