Legally, would it not be the new homeowners property once he had bought the house and it’s still there?[/quote]
Depends on if the property in question is ruled lost, mislaid, or abandoned.
Lost - original owner unintentionally parted ways with the proeprty - finder has superior title to everybody BUT the true owner
Mislaid - original owner voluntarily and knowingly places the property somewhere, but unintentionally loses it - the owner of the house/land/whatever upon which mislaid property is found has superior title to everybody BUT the true owner
Abandoned - original owner has voluntarily parted with the property - finder has superior title