I use a combination, including malwarebytes as well 
Make sure/double check, well EVERYTHING.
Do you have anything like the linked skydrive/google drive and icloud, etc apps.. that they are not trying to do anything (maybe disable them for a day or two, then slowy add one back one by one, see if the usage goes up)
Check things like networkable hard drives, etc if you have them... had a friend blow through usage, as his networked drive, was trying to synck with his skydrive.. wasnt enough space, keep trying over and over and over, etc 
(not sure if kids are in the picture.. but check there too.. and not to say that they are not TRUSTWORTHY.. but check yourself, even if they say they are not doing anything. A co worker, found out his kids, even though they said they were not doing anything.. was hosting a minecraft server.. and it did a good 300g that month mostly from that)