Or someone in range of your Comcast router is using your xfinitywifi network.
Edit: this is incorrect apparently.
Edit: this is incorrect apparently.
I don't have AT&T so I didn't pay much attention to the details. One possible approach: https://github.com/jaysoffian/eap_proxy
It has the same affect on you as your neighbor does.
Bandwidth would be unaffected. Or more exactly, on WiFi (but not Ethernet): as affected as if your close neighbor used that amount (since WiFi itself has limited bandwidth). The connection from the router to the cable modem has lots of headroom, so that would be unaffected, as would wired connections.