I think what you are referring to with shadow networks is the common case that networks have IPv6 traffic floating around, but not in a controlled and managed manner.
This is probably the case in most networks today, as many operating systems, such as Windows Vista, Windows7, Windows 2008 Server, many Linux flavors, Apple Macintosh and many others have IPv6 enabled by default. This means that these devices try to find and connect to an IPv6 network by using Neighbor Discovery.
Even if you do not have IPv6 routing and controlled deployment, these IPv6 messages can be used by attackers to get to your network. So even if you are not working on an IPv6 deployement yet, you should make sure that you secure your network, so these devices cannot be used for attacks.