The past couple of months if I use my phone or my daughter uses her laptop, the home wifi is slow or weak signal. When we check the settings on our devices 'wifi' shows "xfinity" not my home connection.
Did some research and guess what? In order to expand their "free wifi" network they are using the router/modem in my house! Granted it is not the password protected connection I use for my pc's, there is another path it uses for the free stuff.
I found a way to turn it off.

Wow that must have been frustrating to find out, good that you fixed it though!

This is common place in the UK isp's provide routers and they are pre setup up and have additional channels for other customers to connect by, they do need to use there own username and password though. Priority is meant to be given to the standard router tho.

I do not use xfinity modem for wifi, I disabled the wifi.  Then I run a hard line from the modem  to my wifi setup.  All my connection go through my linksys router out to the single hard line to the xfinity modem.  This puts all my hard line and wifi connections on a separate network from the xfinity modem.

I was able to go to "my account" and disable the hotspot.