If you have a bluetooth device that's pinging out to any device, your Pre will get busy with trying to "talk" to it.
I've observed Apple devices that are really nasty for that... Macbook that kept trying to connect to my paired stereo headset, and I've seen my Pre2 be affected by this. (regardless of the devices being paired)
I asked the person to turn his BT off his Macbook to test the theory, and the problem stopped.
I will leave my BT on and turn my headset (ear piece) off during the day as I only use it while walking/driving. As long as no other BT device is pinging everything it can, I've had no trouble.
But sometimes it's hard to tell what's causing a power drain... I've had days that my phone would still be at 80% + , while under days it would get down to around 60% (after a 8.5 work day, plus travel time.. so maybe 9 hours, but with minimal use... a short call or two, a few games, a few online searches, and emails).
As I hate finding myself with low power, I just got another spare Touchstone and now my phone sits on my desk while I work.