I’d probably put out messages on X, bitcointalk and as many other platforms as possible to try and locate the owner of those coins. If they could provide proof that those coins were theirs I would negotiate a bounty and return the majority back to them. I am talking >500BTC for a $1bn rated stack return though.
The best step to take, rather than just taking the BTC without taking any action to find its owner, is an action that cannot be appreciated.
But by making an effort to find out the owner and be able to prove it, it's the right way to go and you can also negotiate how much to return
and you also get the benefit without having to be burdened that the Bitcoin belongs to someone else.
But if you keep trying to find the owner but no one finds it, then it's your own decision because you own the Private key.
This should be the best move if we want some money out of the situation, but the problem is that the owner does not need to give proof, knowing that others have access to their BTC, they just need to move the Bitcoins to another Bitcoin address.
In this case, it may be that the owner no longer has access or lost his private key, and what someone else has found is a backup Private Key that was created.
There are many loss scenarios that can occur, but to claim it requires more proof of ownership.