2 months later has anything changed? Because I'm having a similar issue. The only help I got was them suggesting a password change. And after the initial complaint there was maybe 3 days of minimal usage to make it look like it was fixed before it kicked back up to at least 20gbs a day. I've almost hit 70gbs in a day more than a few times with absolutely no change in routine.
These last few days (of my data cycle) I may as well of had the router off. When the data hit 85% I used nothing but my dvd player and consistently switched back and forth from my mobile data to avoid heavy usage; but even then I had apparently used almost 20gbs a day by my father streaming tv from an underground 3rd party app for maybe 4-5hrs (that we've had for months before this mess started, so it can't be that.) I even unplugged anything that could of been streaming wifi just in case.
I don't have cable, so I use the amazon firestick, like I have been for 12+ months I've used Cox, and this started maybe 4 months ago. Of course, right after my "promotion" ended (that I had no knowledge of) and they had already jacked up my bill by almost $30 without warning.
But before this I rarely ever hit the 85% mark; and my last 2 bills got a very sudden last-minute spike in data usage within the last 24hrs of my bill being due. Just enough to go over. And of course, not without a notice that it had until maybe, at least, 10 hours after the cycle actually ended.