Beyond Frustrating

Take a holiday vacation and go somewhere for at least a day.  Check usage when you get back.

Ensure you have no amazon echos or Dots, or Alexa, turn off ring cameras, or, unplug your modem from 1145pm to 12:15 AM, 24 hrs and 30 minutes, and verify no usage shows. We have devices that constantly use internet in the background that wireshark won't show you. Per it's functionality, - "However, when capturing with a packet analyzer in promiscuous mode on a port on a network switch, not all traffic through the switch is necessarily sent to the port where the capture is done, so capturing in promiscuous mode is not necessarily sufficient to see all network traffic." Unless the device performing the capture is directly inline with all data usage, it won't show it. That means you would need to have your PC run with 2 network cards,  one to the modem, the other to a switch, and run the PC as your router. Then all network traffic  would go through it. Things to remember, your cell phone could be making wifi calls, instead of cellular, like mine does, tablets do auto updates, and cloud backups. Some amazon devices share your network without your consent. 

Check your modem for uncorrectables. They bloat your bandwidth use. For every packet that is loss and has to be resent is that much more data you are using. However, unplugging your modem for a day is a good test if it's something else. Maybe someone cloned your modem? Rare but technically possible. Have you posted logs which may have given your MAC address away online?