If you own something, you must openly avow and say that you own it. That's it. Any property that has not been claimed by a human being, family, or clearly tracked group of humans within three years will revert to the state and be re-sold to pay down the public debt.
Think about it. What does "ownership" mean, if you are unwilling to state, openly, "I own that"? So many problems in the world can be attributed to murky title, from peasants abused by a nearby lord to an oil tanker that befouled beaches in Brittany with no owner ever held accountable, because of deeply nested shell companies.