We have all known about the "unknown systems" for ages, but do we remember if they were discovered while playing or did someone just look into the files? The point is, if they were discovered while playing and no one cheated, then there is the remote possibility that there are more of them out there. Consider - the ones we know about are in RSE, meaning that we don't see them until we are right on top of them. Then, they are in areas of high radiation, which means that rookies will die trying to go to them once they hear the rumor, or we will inhabit easier places. Both are guarded by the two most powerful factions, the Outcasts and the Corsairs, meaning that if we could get to them, we would be killed trying unless our rep was in order. For the majority of us, we wouldn't have been caught dead in those areas, unless we heard about the systems from friends or this site, or the rumor.
So in conclusion there are two or three borderworlds which might still be hiding an unknown system. These systems have all the right conditions for one:
The outer Omega's
2 of the Sigma systems
Omicron Theta
Unless of course, any of you have looked through the files and are absolutely sure that my theory is impossible.

Beware pirates bearing gifts!