I'm having the same problem. It started a few billing cycles ago. I went from using 10 gigs a day to 50 gigs a day according to Cox. The first cycle they said I was at 85% of the 1024 gigs allowed and the following cycle, I exceeded it. They have credited my account for overages so far but it seems that they want me to upgrade my service. We have only been using one hard wired desktop with no streaming or gaming. I've replaced my modem and router, changed passwords, turned the wifi off of my phone and it keeps happening. If I unplug my computer from the modem, the usage stops but like you said, it increases even when we aren't home if I don't unplug it. Cox technicians have come to my house twice and nothing has helped. I'm at my wit's end here.