I have yet to find a single public link network that is undetectable. I doubt what you say because in order to stay undetectable they would have to know how they get detected and I doubt they know that. I have a pretty good algorithm for detecting most sites in a link network. I don't even need public reports from them, I just analyze the links of one of their clients. If I know one single client site that is enough to expose the whole network. If I know more sites, it makes things easier. What I am talking here is an algorithm, something that can be implemented as automated software. If I actually take into consideration that most of the sites in a link network look like obviously part of a link network (you can figure that out generally just by looking at the site) then all you need is a competitor to analyze your backlinks, detect the network and report you.
It is really not that complicated, just basic graph theory and pulling the links from MajesticSEO.
I am sure if they would know how to cover their asses they would probably do it (even though it would increase the cost), but I doubt the know.