Do you stream any media, AND/OR do you have amazon echo/dot or whatever. Sometimes the amazon devices will go into a listening mode and will attempt to identify everything going on, even tv, and it can use a lot of data. Netflix 4K video can use as much as 8gb an hour, (something I found out when I binge watched "The Blacklist". James Spader is an excellent bad guy, lol! Also, do you have a ring doorbell or security cameras? They can sometimes use a lot of data as well. But since you do this for a living, this is all probably stuff you already know, but sometimes we forget, or overlook some things when we are focused on something else. Login to your firewall or check wireshark and see if something happened to have used a lot of data recently?