Note that many ISPs are now using (without permission
) customer's ISP-provided "gateway devices" (wireless router/modem devices) to provide wifi "hotspots" so their customers can wonder around the country and access the Internet wherever they are. These features do not take away from your bandwidth, and those connected users cannot (yet) access your network or your connected devices. But they are using your electricity - though admittedly that is probably pennies/year.
That may be what you are seeing. You may be able to disable it in your gateway's admin menu and see if it breaks anything on the wireless side of your network. Make sure you can access the menu with an Ethernet connected device before you disable the wireless side or you might lock yourself out of your network. That would not be good.
