I am afraid that the scenario as presented in Leviathan has not passed the sniff test for years.
The Spinward Marches as a distant frontier of a decaying empire where dissent was rife and the worlds across the border unexplored you could make a case for having a few unknown subsectors to explore.
A Spinward Marches that is no longer the true frontier and has been explored and settled for over a thousand years makes unknown worlds just across the border preposterous. This is not Mongoose;s doing, this was GDW changing the setting.
You also get this effect in Mavuzog as I am discovering; it is key to making DNR work since you need to get into unexplored space. It raises questions and problems though, since although ISS may not have info, locals surely would.
TBH, I'm discovering this is also sort of the problem for the Mavuzog & Harea sectors, which are right at the edge of Charted Space, so you end up with a sharp line - long settled space with some high population systems on one side of the arbitrary line and suddenly no settlement at all on the other (indeed, nothing at all there).
As the DNR makes its way into Incognita, I've decided on two things to make it work:
1) the sector line is a far as the ISS has gotten just because that was the mission brief, but there are actual settlements further out. However:
2) settlement does indeed peter out, and there is a strong fear instilled in the cultures of the Theta Borealis and Mavuzog about going spinward. Rumors and half-baked explanations abound. Where this fear comes from is an interesting question. Ships go missing, so do distant settlements. Maybe the crew of the DNR will find out why? Don't worry. I'm sure it will all be fine.
No such "falling off the edge of the map" applies to Foreven and so having it be mysterious is just a problem for the Referee.