[Brando]

It is possible by just streaming (HD) you can be using that much. I think I read that on average 1 45-minute episode of a show can be almost 1GB in size on Netflix.

You could also have "smart" devices connected to the internet, doing their thing. Also, mobile phones (iOS especially) can consume a metric shyt-ton of data once on WiFi.

It might be time to put your "smart" devices on a segmented network with no internet access and see what gives. Also update your WiFi password and see if any neighbors complain. If you have guest access turn that off.

Oh, and one last thing I found out. Providers like COX, Comcast, and Xfiniti have integrated to their Wireless AP/Router combos an open guest network. It's designed for other subscribers to have WiFi access wherever they go. If you are using a router/modem combo from your ISP, toss it and buy a separate Modem and separate Router.