Cox's data usage monitoring is checked by a third party for accuracy. it's 99.99% accurate. That means something IS using data. Do you have security cameras? Do you have amazon Alexa, or Echo devices? (They can go into a loop using all kinds of data as well as share your data with other people that have those devices). Do you have ONEDRIVE on any mobile devices? It can backup daily your entire device. If you have the Cox app, you can monitor data usage, if you have the pano modem, I think it can break down the usage as to what KIND of usage it is. If you use a VPN that can be seen as network monitoring in the data usage.
Also, no matter WHAT router you are using, Cox pano or your own, it can tell you every device connected to it.