Ok,
First lets assume that you've isolated that a button wasn't being pushed the entire time it was in your backpack.
Second lets assume that you didn't accidentally have Pocket Tunes or some other media player running in the background, unknowingly.
Third lets assume that the battery was fully charged and that this was confirmed visually before you put it into the backpack (lets not assume that we just left it in the cradle or connected to the cable assuming that it was charging). The Athena connector presently used since the T5 is somewhat tempermental in that it may be connected but not completely and as such gives a false sense of security or insecurity as in my case since my T5 was in similar circumstances.
Then here is what I'd suggest:
There will be a software program that could be the culprit. The most likely candidates are as you already mentioned followed by backup programs that write to the SD card, media players, bluetooth based programs or even the BT by iteself if it's set to discoverable then it's burning the proverbial bridges of electricity as it tramples over them to find that elusive headset that walked past you on the trail in the other direction.
So there are two ways to proceed: Either off load all software and one by one add it back checking the battery status as you go. Or off load one program at a time (using some deductive intuition to "think" which is the most likely culprit) and checking battery life as you go.
A more radical approach - throw the unit into the sea and go back to pen and paper........but then you'll never know the cause......so I doubt that this last suggestion is viable.
