That's my question. A person who lost his private keys for years and recovers them. Maybe he mined btc or he won btc with a faucet, how can he prove all that after 10 years ?
Don't worry about that person. If he is someone who has a lot of money, he will have no problem bringing it to the surface. The problem would be for you if you suddenly recovered a few dozen bitcoins that you had on an old laptop from when they were worth pennies.
We have discussed this many times in the Legal section of the forum and the conclusion is that the best thing to do would be to hire a professional specialized in these matters to advise you, as what you can do will depend on the legislation where you live.