Heathcliff74 (of XDA fame) spent a lot of time under the hood to figure out what was causing him to have absolutely massive amounts of data (over 600mb in a day) and severe battery drains. This is worth a read to see the level of investigation used, but the outcome is simple – it was an app with a live tile and the server the tile was using to fetch an image was unavailable. This apparently causes the tile to endlessly loop trying to fetch the image it was seeking. It’s also notable that he did the same test on a WP7.5 device and it was not reproducible so this is a WP7.8 issue (he does not know if WP8 is afflicted).
His write up is worth a read. Hopefully Microsoft will take note of this and issue a patch asap.
So you know, the app in question is the unofficial tweakers.net app which you can find here.