The field of metaethics is broadly concerned with the following questions: are there any true moral facts? And if so, how can we come to know them?
As an example of what this would mean: take natural-rights libertarianism, as espoused by Robert Nozick, Murray Rothbard, and legions of spotty teenagers. According to this theory, there exist certain facts along the lines of the following:
(a) The copy of Anarchy, State, and Utopia upstairs is my property.
(b) For any entity X, if X is my property then others ought not to interfere with my usage of X unless my usage of X interferes with their usage of an entity Y which is their property.
Of course, a lot of attention in this kind of theory will be devoted to exactly what it means to say that an entity is someone's property. A standard response made by a non-libertarian philosopher would be to observe that the notion of property is entirely socially constructed. To bring out the difference between socially-constructed and non-socially-constructed features of things, compare the properties of belonging to a person and of being less dense than water. Whether something belongs to me or my neighbour is determined entirely by the beliefs of society: if everyone believes the copy of ASU upstairs belongs to my neighbour, it's not that everyone is wrong - it's that the book actually is my neighbours, and I will be obliged to return it to him at the next opportunity. If something is less dense than water, however, it matters not one jot what any of us believes - it will float, and all the assertions in the world will not change that.
Since property is socially constructed, then, perhaps we ought to construct it strategically so that it operates to the greatest advantage of all. Thus we might decide to agree that notions such as taxation are baked into the very notion of property: taxation is not theft, but simply the proper functioning of the property system. (There's a more ambitious version of this argument which holds that no property would exist without a state and so submission to the state in general is part of what it means to own property, but this is silly because (a) property has existed throughout history without the existence of states and (b) even if that were not the case, it's not at all clear how the move from an is to an ought is supposed to be occurring here).
One thing that would support natural rights libertarianism, then, would be if facts about property somehow turned out not to be socially constructed but to be intrinsic features of the world in the same way as density. It turns out that there is a well-known fictional universe in which this is the case: the Harry Potter novels, in which a key reveal towards the end of the last book is that the Elder Wand, a weapon of deadly power, never truly recognised Voldemort as its possessor - despite him having wielded it for much of the last book, ever since he ransacked the tomb of Dumbledore, a previous owner of the Wand. Instead, the wand recognised first Draco Malfoy and then Harry Potter as its true owner, despite neither of them having prior to this point even touched the wand. In the Harry Potter universe, ownership is not a social construct but a real and tangible feature of the universe - and so it may well be impossible, even if desirable, to move to a more socially beneficial meaning of the notion of "property".
Libertarians should not rejoice too quickly, however: the way the wand passes between owners almost always involves violation of the Non-Aggression Principle. Grindelwald stole it from Gregorovitch, Dumbledore kept it after defeating Grindelwald, Malfoy ambushed and disarmed Dumbledore, Harry burgled and overpowered Malfoy. While there are substantial facts about property, which stand in addition to the facts which are known through science and empiricism, they are surely different from the facts which libertarians would have us believe. Perhaps not entirely different - wands aside, most objects seem to behave much as they do in the actual universe with regard to owners - but not the same either.
As a final aside, it is interesting to note that this universe also contains one of the more notable examples of a society with markedly different but non-utopian rules concerning property. I refer, of course, to the goblins, who believe all objects to truly belong to their makers: one cannot purchase an object, only rent it for life. To pass on to one's heirs something that one did not produce oneself is regarded by goblins as theft. Unless the original maker of the Elder Wand is still alive (and according to tradition, the wand was in fact made by Death Himself), this theory must surely remain live as a possible metaethical truth about property in the Harry Potter universe.