While this is of no help to you unfortunately, I have just experienced this myself this weekend.
I received 3 sudden texts early Saturday morning notifying me that I had gone over my 1.25 TB limit. I use the internet constantly both for work at home and personal use, but rarely over go over 20-30 GB in a month. Now it is Sunday, and I am 200 GB over for no apparent reason. I have also run virus scans out the wazoo, checked services and processes on both of my computers. Nothing, everything is normal except for this outrageous usage coming out of nowhere. Then there is the fact that Cox Tier 1 is useless, and I have to wait until tomorrow to even attempt to get to a tier 2 tech while "using" God knows how much more data.
This is much more than a simple issue with someone's computer, this is a networking issue that COX needs to investigate and resolve.