Apparently 69,985 found Raxxla and so disappeared from view...I think the other 15 have been found. (edited numbers for accuracy)
Seriously, it depends on how well scanned the systems were/are and how good the tools were at the time and how well used. I use the FSS religiously but when there's 50 odd undiscovered signals I hardly ever put it into Megaship range and scan around, I just assume USS/NHSS and move on. Point being the 1st one was found by flying 10K ls from the last planet towards another star system, following a seemingly meaningless message. Would I have found this using FSS in the normal way? Doubtful I think unless it resolved itself, I rely too much on the POI in the Nav Panel after scanning all the planets/asteroid belts.
In undiscovered systems I investigate signals more closely but where I expect loads of signals (and no Nav Beacon to resolve them all) I rarely check them all out or even look. So depends on whether they would all show up as POI and how close I would have to be to discover it first...I'm not entirely sure undiscovered Generation Ships, Raxxla or the DW station would show up in POI just from initial scans so work on the principle they won't until proven otherwise.
I've found unmapped planets in Aisling Duval space in the last 2 months, including one with a mapped 8H planet but no other. implies old ADS and nobody actually flew out to the other planets/moon since so a megaship could be there...or anywhere really, the bubble is by no means fully explored, not systematically.
I've heard interviews with DB where he mentions the 70K figure and to me it just does suggest theres a few out there, even if 99% of them crashed/blew up/mutinied/left no trace somehow, that still leaves a lot...and some might have made it...all the way to the Col Sector or other locked ones for example.
Its far more fun to believe in mystery and treasure to be found and DB et al have been brilliant at this for 30 years, some of us are still waiting for the Graveyard and that's an easy one to implement visually surely but then that would just be it, its there, next. So I choose to believe there certainly could be more out there, but that we may not find them, but I hope I do, it would be truly awesome, if I don't, the mystery and treasure hunt lives on...win/win.