To further increase the chances of owner noticing the thread, I'd spend the output to the same address with a proper OP_RETURN note saying:

It'd come as an instant notification to their wallet whenever they decided to open it. If they have a backup, they will immediately send the funds back to one of their wallets. If they don't, they'll try to convince they're those who owned it.

However, if there isn't sufficient evidence, such as a writing style or possible lack of color, which are things you haven't revealed to us (and should not!), the real owner won't be able to convince you. If I were you, after all, I'd keep it in the same way I'd keep cash if I found it on the street.