Is this a matter of law or NASA policy?
Looking at it from the other side, the US government spent billions of dollars to go to the moon and bring those rocks back. To whom else could they belong? Who else could claim any right of ownership, except in the one case we know of where NASA accidentally sold some dust?
The official gifts to the states and other countries were the property of those jurisdictions, who didn’t have the right to dispose of them. The researchers who got samples not only didn’t have any right to dispose of them, they also agreed to rigorous audit requirements regarding the location and status of the materials.
In looking for some documentation of that process, I found this document: “NASA’S MANAGEMENT OF MOON ROCKS AND OTHER ASTROMATERIALS LOANED FOR RESEARCH, EDUCATION, AND PUBLIC DISPLAY.” [pdf]
I’m just about to sit down to dinner, so I don’t have time to read all 35 pages, but I wouldn’t be surprised if somewhere in there it states the legal justification for NASA’s ownership.
I’ll read it when I can and check back a little later to see what you guys find out. Then we can fight about it some more! 
No fighting from my side. I responded to a post that made a pretty definitive statement and was looking for a source of that information. I’m going to bed now so let me know what you find
Why don’t foreign countries have the right to sell their rocks?
US law shouldn’t be able to prevent foreigners from getting their rocks off.
In case of an astronaut picking up a piece of rock for his family, I could perfectly see an argument that in doing so, this astronaut acquired good title to the rock. The counter-argument would have to be that in picking up the rock, the astronaut acted as an agent of NASA. But does this argument hold if picking up the rock was not part of the atsronaut’s mission? It takes only a second, after all. Then he would have done so on the occasion of carrying out his duties on the Moon but not as part of these duties.
FWIW, in the aftermath of the Apollo 15 stamped envelope scandal, NASA retained those envelopes still in the astronauts’ possession, but a few years later (according to the Wikipedia article) a Justice Department analysis concluded that NASA probably had no legal basis to do this and violated the astronauts’ constitutional rights. I know the case is not directly analogous to ours (because the envelopes had been carried from the Earth to the Moon and back, not picked up on the Moon). But it’s the closest real-life incident that has been extensively discussed. And it does illustrate that NASA making a legal assertion, and that assertion actually being correct, are two different things.
[Moderating]
While the question of ownership of moon rocks is a fascinating one, it’s not precisely on topic for this thread, which is about items of unknown location. Perhaps a new thread could be started for it?
So, back to the original premise of the thread, may I suggest the missing final panels of the Bayeux Tapestry?
At least two panels of the tapestry are missing, perhaps even another 6.4 m (7.0 yd) in total. This missing area may have depicted William’s coronation as King of England.[26] A poem by Baldric of Dol describes a tapestry on the walls of the personal apartments of Adela of Normandy, which is very similar to the Bayeux depiction. He describes the closing scene as the coronation of William in London.[37]
Granted it’s more of a “possibly” rather than “probably” still out there - but it’s a part of a factual item that existed at one point with no clear record of it being destroyed.
Abandoning Moon rocks and returning to missing artifacts … almost.
I wonder why you suggest that?
Other than both being designed to traverse open snow and ice, there is nothing in common between the design features of Admiral Byrd’s Snow Cruiser and the Soviet Kharkovchanka. Which greatly resembles a bog-standard armored personnel carrier and the Snow Cruiser not at all.
Compare and contrast with the US’s roughly contemporary M113 armored personnel carrier - Wikipedia. Yes, the Kharkovchanka is both wider and taller. Because it was not constrained to fit into a C-130. But otherwise pretty similar.
My bottom line: IMO the Kharkovchankas were overgrown APCs, not copied Snow Cruisers. Unlike say the Soviet Tupolev Tu-4 - Wikipedia which was transparently a copy of the US Boeing B-29 Superfortress - Wikipedia.
I intend to do so, based on the reading I have been doing on the subject. It may take a few days.
The missing parts of the flag that flew at Fort McHenry.
Several feet of fabric have been lost from the flag’s fly end, from cuttings that were given away as souvenirs and gifts, as well as from deterioration from continued use.[6] It now measures 30 by 34 feet (9.1 by 10.4 m). The flag currently has only fourteen stars—the fifteenth star was similarly given as a gift, but its recipient and current whereabouts are unknown.
Those would be very cool I concur, but since no record of them being in existence for over 1000 years, I have little hope.
In all the years I was dragged to Fort McHenry, this is the most fascinating thing I’ve learned. Thanks!
Maybe not the looks or the size, but the mission.
Cat tracks are FAR superior than slick tires, by far.
The missing sixth painting from Pieter Breughel the Elder’s “seasons” series.
I came across this similar reddit thread today:
What treasures that we 100% know existed still haven’t been found?
From the reddit thread:
We can remove this item from the list.