Google doesn't have complete knowledge of the return path to the user.
Too be honest - upon further thought I don't think they would even know that the user is behind a wireless router straight from the HTTP information in the exchange between them and the user. The IP information just isn't enough because the DHCP leases handed out by the ISP doesn't have anything like GPS or street address information in it. PLUS Google doesn't have the DHCP lease information and the ISPs don't have any reason to tell google who is where. The DHCP is just a mac address of the end station connected to a cable modem or DSL modem. In our cases generally that would just be the outside interface of our home router. Again - from that you can't tell ANYTHING about the interior network. Not the number of hosts, not how they are connected - nothing. Just that there is at least a single host and they are requesting stuff from the internet and what not. You also can't tell what the interior hosts mac addresses are because in the path of the packets the requesting MAC addresses (the source mac) is changed to the next hop router's mac address along the path. In the HTTP packets however there may be some 'stickiness' defined based on the mac address or ip address (in that case it would even be the internal ip address not the outside public).
I could provide diagrams and packet captures if you like.
With all that in mind - I have no idea how they know where you are.
