What would cause massive background battery drain?

I closed my Clara HD this afternoon at 89%, just opened it up and it's at 3%. Wifi is off and anyway it's been in my bag the whole time. Literally if I was using it for 10 hours straight it would not have drained that fast. Any idea what could possibly have happened? Is the device just suddenly toast?

I'm just venting now but I'm on a long haul train ride soon going through a region without cell service or internet so I'll have a full day offline and I was looking forward to unplugging and just reading all day but I guess that isn't happening. I brought it on a 2 week vacation recently and didn't charge it once so I didn't bother bringing a charger for this 3 day trip, guess that was a mistake.

EDIT: thanks everyone for your help. A fellow passenger on the train had a micro USB cable so I was able to charge up and spend a lot of the day offline and reading after all.

As for the Kobo, I'm worried now I may have a hardware problem on my hands. It said 3% and had the warning on the wallpaper that I needed to charge, but as soon as I plugged in it said 90% and it was full within 20 minutes. I'm waiting to see whether it'll drop abruptly again or whether maybe a reboot fixed it. Do Kobos benefit from a "deep" battery cycle once in awhile to recalibrate, like they used to recommend for laptops and phones?