The only way that a Big New Handmade By Frontier Discovery could certainly be only found by one player would be if it was in a permit-locked area that has been locked since the game's start, and FD suddenly unlocked it.
Mind you, some of the unknown permit locks have only been introduced well
after the game had launched: if Frontier unlocked those and added new stuff there, the body might already be tagged anyway. For example, I have a tag on an ELW in the NGC 2286 sector that I can no longer access, as it's now locked.
If Frontier will ever open up a locked area, people will be swarming all over it of course: the opening itself would be a giant clue that there's something in there that they want us to find.
But since they have a clear tendency of placing new unique content close to the bubble, this means any unlocks would at most be (one of) the permit locks around Barnard's Loop, and not the distant locked spheres that are all five digits of lightyears away.
The Zurara was an exception, but it was said to be in-game since launch. Speaking of which...
For some actual examples:
[...]
5) The Zurara.
Really obvious once you're in the right system; known to have been there from the start; genuinely no-one had entered that system because the locational clues were vague enough for the first few years.
It's not just that they were vague, but also that there was only one clue was in the right direction enough (the line between Reorte and Riedquat), and a bunch of others that were either in a direction too far away (this including how the Formidine Rift settlements were on a line between the Heart and Soul nebulae and the bubble), irrelevant, or even false. (Like "Whether it's found depends on who goes looking." - oh, come on.)
Perhaps it would have been found if the Reorte-Riedquat line was the only one given. If all the Commanders who took part of the search were concentrated on this line only, I think it might have worked... but well, that's not how things ultimately went. (Mind you, for the better: the discovery would have been less impactful if it were the original incarnation, and only later patch notes would have said "the Zurara is now a wrecked megaship and we added voice-acting to the logs.")
Of course, there's at least one way Frontier can side-step some of this: if they place something on a thin atmo. body's surface, then we can know that the earliest it could have been there was with Odyssey. For all that would help.
But if they hand-place a new location, or locations, and not procedurally generate them, then it should be trivially easy for them to pick systems that haven't been discovered yet.
However, if somebody went and missed something important, as per the first post's question, then is it really their fault they missed it if the game buries the alert in a place rather easy to overlook, or is it mostly the fault of poor design? Personally, I'm leaning towards the latter.
Take Guardian Ruins, for example. When scanning planets, people tend to look at the top right corner because that's where the signals are to be listed. Except for Guardian and Thargoid signals, as if they are present, then the notification for them is in a different area, under Locatoins, lower down. Granted, at least now the text is in yellow there, but it might still be easy to miss. I know at least one time this actually happened: the guy who assigned boxels to players in IGAU to search scanned a body in his boxel, and missed the ruins on it. (Just to make sure: I'm not talking about myself here. While I came up with the idea that the area might be worth searching for Guardian ruins, I wasn't the one who organised the squadron survey.) So that's a motivated player specifically looking for that exact type of thing, and still accidentally missing it... because with that interface, that's easy enough to do.
So, if you found something important enough, in my opinion the game should point it out even more than it does for the "usual" stuff. Certainly not less.
