How so? If someone loses their paper wallet, you can open threads in Internet boards. Or send some funds there with a message, describing where and how you found it.
The owner might not browse those internet boards or be much active on bitcoin-related discussion boards at all. You can take some funds out and return the same coins with a message, but the owner might not be looking at what is happening on the blockchain. The coins can be someone's long-term cold storage that will only be accessed years down the line.