Mine has done this kind of thing twice. Last night was the second time. I was at a place that is a poor signal area for Verizon around here (yes, I have complained about it before). I used the phone a little, signal was going between 4G / 3G / 1X / None and voice bars 1, 0, or occasionally the little "x." Put the phone away, dozed off, about 3 hours later, down to 50%. Finally put the phone on charge around 4:00 a.m. at 16%, at 7:30 a.m. only up to 23% and very warm.
I normally use the Sense desk clock as my alarm clock.
It's currently turned off, still has an orange light, but is cool to the touch. I run Watchdog, and it has flagged nothing in both instances.
Both times this has happened, I have noticed that Angry Birds kind of "stuttered" while playing, so SOMETHING is going on.
EDIT: Battery was at 99% when I turned the TB back on at 9:00 am.