Millions of bitcoins are considered lost. Who will find them?
I think there is no way you can find a lost bitcoin. Bitcoin can only be use digitally and never exist in physical form so it would be difficult to find any lost bitcoin. Once it was lost or stolen it will be lost forever. That's why we have to be careful on copy pasting our private keys. Once you mistakenly put your private keys anywhere here in the forum, well good luck to you and your wallet.
Lost bitcoin can be tracked, but it cannot be retrieved back until the receiver end has the mentality to return it back. I've a personal experience in it. Got hacked and around 0.5btc was stolen, but I have trying hard to get it back. Only thing I was able to do was to see the transaction happening to different wallet addresses.