Did you know about this? Your modem may be broadcasting a independent wifi signal for others to use. Comcast claims it's independent of your own wifi home network but... Who wants to take the chance...
I checked on mine and it was broadcasting a xfinitywifi ssid. I was able to connect to it with ease but didn't go any further. I logged onto my account and went through the steps to disable it and it's now disabled.
So, those cheeky bastards!!!!