If you use a wireless Comcast router, there's a very good chance that it's functioning as a public hotspot for any Comcast customer wandering by. It needn't be that way -- I've got instructions for killing the public hotspot while still keeping the router for your personal use.
Few people realize that their wireless Comcast modem/router combo does double-duty -- not only is it a home wireless network, but it also provides other Comcast customers hotspot-based Internet access when they pass by. If you've ever seen an Xfinity hotspot appear on one of your wireless devices, that hotspot is almost certainly tapping into someone's home Comcast router.
The Comcast routers have two different antennas, one for your home network, and one for an Xfinity public hotspot. So there's no evidence that it's a security hazard. However, it does mean that you'll be sharing your bandwidth with other Comcast customers.
It also means that you're footing Comcast's electricity bill. That second antenna for the public hotspot uses up electricity, and you're paying for it. The tech company Speedify claims that it costs each Comcast customer $23 per year in additional electric costs. And two San Francisco residents are suing Comcast over the dual use of the routers, claiming that Comcast is "exploiting them for profit" by using their home Comcast routers for the service. The suit charges that Comcast hasn't adequately told its customers about how the routers are being used, and alleges that "the data and information on a Comcast customer’s network is at greater risk" because of the way in which the router "allows strangers to connect to the Internet through the same wireless router used by Comcast customers."
If you're a Comcast customer, there's a quick and easy way to turn off hotspot access to the public. First log into your Comcast account at customer.comcast.com. Then click Users & Preferences-->Service Address, which will have your address. Underneath your address click Manage Xfinity WiFi, and then click Disable Xfinity Wifi Home Hotspot. After that, click Save.
If you're feeling a little more adventurous, you can disable the router's wireless capabilities altogether, only use it as a cable modem, and yse your own router instead. For instructions, go to this reddit page.