I get this on occasion - sit with it charging whilst messing on the computer of an evening, then disconnect and goto bed - i disconnect wifi everynight because i *know* thats a problem with me personally.

Random mornings I have woken up and its dead.

There is a process running amock and eating CPU for breakfast, its not being killed by the watchdogs or anything.
And its not a corrupt card issue because it would be a problem all the time.

I've attempted to run top in the past and log output but it doesnt seem too good for the purpose (it could be a low but constant cpu munching, or not cpu at all, maybe wifi chip or BT chip or gps internals or something).

My best guess would be an application run earlier in the evening didnt cleanup and close everything expected which caused the drain.

Further investigation would require the oprofile kernel flags to be set (or built and flashed) and actually monitor whats happening overnight.

Lots of work though for a random rare (but annoying) occurrence.