It will be a dilemma when actually finding other people's keys that have thousands of bitcoins. But would someone be that careless with a wallet of thousands of bitcoins or other amounts. Or because of some phrase incident, the seed was lost and found by someone else.
If I find it, the first thing I will do is check the transactions that were made. checking whether someone legitimate has the address or they are looking for it. i will actually tell the owner if i find it. Not wanting to be hypocritical, maybe I can get a few percent of the assets I find.
Feel sorry when someone loses their keys accidentally with quite a lot of assets. Just imagine if it happened to ourselves, of course we would be very frustrated and try to find out whether other people held the seed or not. But the worst thing would be losing the seed and never finding it, it would be immovable asset.
I would send this amount to some charitable foundation for sick children or children without parents.
But it's possible that the real owner of the fund has another copy of the private key and can still spend the fund.
It may be better to wait or make a transaction sending a small fund to inform the owner that the wallet is compromised.
good suggestion so that the owner of the wallet will know if someone is trying to tell that a seed was found and a transaction was made. The actions taken by @lovemayfamilis are indeed good actions, but it's better to wait for the owner to respond first.