I don’t see how you could ever get a definitive answer to your question. We really don’t know what significant finds are kept secretly in private collections. To be honest, even a small and unassuming find could be of some importance but as soon as it’s placed in a private collection the evidence is gone.
I follow a very well known treasure hunter on YouTube, he never reveals the location of his finds, I once watched him retrieve a hoard of civil war cannonballs, weapons and associated paraphernalia from a shallow river, he surmised that the finds had probably been lost during an attack on a number of wagons that were carrying supplies across that river.
My point being that the area was obviously of some historical significance but the location was not revealed, Ive no doubt that he’ll continue to clear the area of finds and maybe he will eventually report the artefacts along with the location but the damage has been done because the finds can not be professionally recorded in situ, the only thing that anyone will ever know is that something happened at the location but the all important clues have gone.
Here’s one of the people that I follow, he’s a responsible treasure hunter but just look at the history thats been removed.