No it doesn't
PLT devices have discovery protocols (by what looks like a periodic broadcast) so they can see each other. Chances are they also use uPNP and are probably visible to the HomeHub. That's the beauty^H^H^H^H^H^H danger of uPNP.
Even if they do not use uPNP, BT can probably make a reasonable guess about whether such devices are on the net by sampling the packets on the net, and looking at the first six octets of the MAC address that identified the vendor of the device.
My PLTs are Intellon based, and come with a (Windows) utility that allows you to set the encryption key. Not only does the utility find the devices, but also can tell you how fast they are operating, so there must also be some other magic under the covers. I have a Linux utility in source, so I'll have a look at how it works.
Still, I have a Linux based firewall (really, separate from any of the comms kit - Smoothwall as you ask) between my ADSL router and the rest of my network (yes, yes, I know that there is a risk that the PLT escapes onto the wider electricity network, but that's why I set my own key), but it means that my ISP cannot probe my network.