That is not how Comcast's wifi works. You'll find that when you connect to an Xfinity Wifi network in a subscriber's home, you're on a completely different network from their home network.
How much would you bet against that when the cop 'determined through his "investigative resources"' the ip address of the guest Xfinity network still came back as 'associated with Jones's Comcast account' ???
Are you claiming that the Comcast modem uses a different IP address for Comcast-network side of the NAT? That the modem has two routable addresses, one for the 'house', and one for the 'open' world?
Yes, that is generally/approximately correct. I'm not sure precisely how it's implemented on the public side, but you're placed into a seperate VLAN that enforces the captive portal. Traffic from the public WiFi network is isolated from the subscriber's traffic.
That does not mean it uses a different public IP address that communicate with the rest of the intermet, I have seen dozens of such public/private wifi systems, abd never seen one with a separate public IP
That doesn't at all answer GPs question though. That they're isolated LANs doesn't at all answer what WAN address they use. If no other indication it probably uses the same.
I have multiple VLANs at home, and I only have one WAN IP.