If you are using Win 8 the live tiles can be your source of data usage. You can right click on the active tile and there should be an option on the bottom tool bar to turn off the live tile. Not all tiles use data but if you have one with current stocks, weather etc updating, they will eat data. Even if you think they are all off, I would go through them one by one to be sure.
I guess I'm the odd man out, but I've been using Win 8.1 in the office on a 27" all in one for a couple weeks now and I really like it.