If nothing was spelled out beforehand (either verbal or written contract is equally enforceable), then here's the law: It belongs to the property owner.
There was a case of a construction renovation worker who was working on someone's house. While they had the kitchen sink torn out for repair work .... found a hidden cache of old coins that had been secreted into a hiding compartment back in behind cabinets. They were obviously hidden many decades earlier, as it was an older home. He showed the homeowner. At first, since the two men knew each other, it was just "assumed" by the finder that they'd split it. But as revelations unfolded on the value of the find (they began to look up dates, count it out, and deduced this was big money), then the owner of the house changed his tune, and was going to keep all of it. Even though he's a) not the one who found it, and b) the finder could have kept his mouth shut, yet c) the homeowner still owns the homes, hence all the contents in and on the property.
HOWEVER, even though the above is true, yet I have never bothered to spell out ahead of time any contractual split percentages, etc... I just usually throw out a casual "you're welcome to anything I find". And most all property owners just tell you to "keep it". Because if you start showing up with contracts, and legal talk, percentages, and so forth: All such talk of that is certain permission-killers. Just conjurs up images of legal hassles, etc... Or that you're somehow "certain to find tons of $$", etc... So I have never bothered with anything above casual. Or, heck, just telling them they can have it all, d/t this is just a hobby for fun of mine. And again, I have YET to have any property owner not let me cherry pick a few things back out for my own collection.
