Totally at random I came across this today. It may be old news to some, but it's new to me. And I don't like it.http://www.comcast.com/wifi/hotspots.html?SCRedirect=true
If you have an Xfinity modem with built in Wifi, it may be offering a free hotspot to anyone who comes along. In theory, this is great for when you have company over and don't want to hand out your password 50 times, give access to your local network, or you believe in sharing everything.
Not me. I can be a greedy prick some times.
Now, granted, people who want to use the hotspot need to already be Xfinity customers and have a special app. But that's beside the point. My pipes are mine damn it!
To disable it you can either call customer support, or follow the steps listed here: http://forums.comcast.com/t5/Home-Networking-Router-WiFi/How-to-disable-XFINITY-WiFi-hotspot-on-your-modem/td-p/2144958
Be aware that if you have to reset your Xfinity modem for any reason, you may have re-enabled the hotspot, and need to turn it off again.
Get your own internet you hipster moochers!