Scanning Private Keys
All bitcoins are secured behind private keys. If you know the private key associated with a bitcoin address, you can spend that bitcoin.
Only a finite number of private keys can exist. But the number is so large, it's impossible to view all of them. This is what allows Bitcoin to be secure.
One website, directory.io, lets you view all bitcion private keys page-by-page. Click any address to see if it has a balance. If it does, you can use the private key listed to keep the Bitcoin.
There are so many pages that you will never see them all, but you can try!