If you removed AC power from the modem then your reported usage will not be correct due to the modem not being able to sync with the Gateway.
If the modem was left powered on and the Ethernet cable was left connected then we need to look at other items:
> Is WPS disabled in the router?<
>Is Remote Access disabled in the router?<
>Have all Guest Accounts been disabled?<
>Has the default username and password been changed?<
>Has strong wireless encryption been enabled on all wireless frequencies?<
>Have you audited the permissions afforded devices such as printers?<
>Are all devices .. both wired and wireless .. truly "off" and not just in hibernate mode?<
Do you have a router that tracks data usage?
For a better understanding on how data can be used see this topic: