By a strange coincidence, I found this yesterday as well
I'd reflashed my HG612 with the Asbo formware, as I'd noticed that upnp wasn't working on the later Wolfy firmware for this device, even when "enabled" via the interface.
Once up and running, upnp was working, Windows saw a new device on the network, starting a program that wanted ports open would automatically open ports (I know, I know...
).
Later on I booted a different Windows 7 PC and was surprised to find a "Teredo Tunneling" port open to that machines internal ip address!
I too found it was Microsoft TCP/IP V4 > V6 thing, but hadn't yet done anything about it, so thanks for the info on how to disable it!
Ian
ps at least having upnp enabled and working properly, you can see which ports are being opened "behind your back" by which computers on your network