For example, send a small amount to another address and return it back with a signed message with information so that the owner moves btc to another address. It is not a fact that the owner will immediately detect this if he rarely checks his wallet and it may turn out that he has no other access due to the lost paper wallet, but it's worth a try.
As explained above, a better solution would be to make a transaction sending a tiny amount of coins to the paper wallet address, and including an OP_RETURN output with some information on how to contact OP, perhaps with a link to this thread. This prevents any issues in the rare but possible case that the true owner has a signed transaction involving the original output stored somewhere.
Whoever the original owner is or who their successors are, they have lost a physical object that used to be in their possession somewhere and somehow. Hopefully, that's not their only backup, they will discover sooner or later that it's missing and move the coins elsewhere.
Yeah, that's a good point. I also have a number of back ups stored off site, as everyone should. These backs ups are obviously highly secured and incredibly unlikely to end up on a beach somewhere (not to mention that no single back up is enough to compromise any of my wallets), but at the same time, I obviously don't check on these back ups every day. It is possible, however unlikely, that I would lose one of my back ups and not realize for a period of weeks. There is no telling how diligent or otherwise the true owner of these coins might be, and it could very well be months or even years before they realize that their paper wallet is lost.