Similar problem here. Had it in my bag charging overnight, found it in the morning with the fan whirring :-(. disabled keyboard-wake.
Then it dropped 30% while in my bag for 5 hours. Then I used it for an hour, leaving it at 50%. Just opened it up again several hours later and at 30%. Completely pathetic. My SP2 never did this.
It was charging inside a bag? You should charge it in a ventilated area.