I experienced this a few months back, and as usual, cox was of no help. Nothing appeared unusual on my router or logs, and the data didn't match. Cox has no traffic analysis available to verify or validate against. I usually max out at 30GB a day, but during this issue was seeing 70-80GB a day. Eventually it "fixed" itself. Once again, nothing changed. Not sure if it's related, but my modem died a couple of weeks later.
I wonder if the modem was experiencing issues and began "beaconing" thereby spiking the data?