Anybody could report some bitcoin as stolen and make a claim on it at any point in time if the private key for that bitcoin ever became public or was cracked.

Anybody could report some bitcoin as stolen and make a claim on it at any point in time if the private key for that bitcoin ever became public or was cracked. All you have to do is come across a private key of an old account with no bitcoin in it and then announce to the world that it was your account and it was stolen and the money should be returned to you regardless of who has it now. Also, any transaction can have multiple inputs and outputs. If your btc is stolen and combined with 10 other accounts and then funneled into 37 completely different accounts, you now have some percentage rightful claim on 37 different accounts. If each of those 37 create transactions get combined with other bitcoin inputs and have more outputs you could end up with some dilute percentage of all bitcoin accounts holding some claimed stolen coins.