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.
Just a thought: you could add a "canary": a very simple unencrypted paper wallet with private key QR-code and instructions how to claim it. If someone finds it, you lose the small amount, but you'll know your backup is compromised without driving into the desert to check and digging a hole at N34 59 20, W106 36 52 each time.