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.
Joke's on you, that's where I store my barrels full of millions of dollars, not my bitcoin back ups! Wait... 
That's not a bad idea, actually. However, I am more than happy that all of my off site back up locations are incredibly secure and are highly unlikely to be found by anyone either deliberately or accidentally looking for them, though. If any of the back ups were to be compromised, it would far more likely be due to a massive natural disaster, in which case the canary wallet would not help (although I would probably be aware of the fact via news reports or other media). And in the case of someone deliberately looking for my back ups, they would know not to spend the canary and attract attention to themselves, since as I said after finding one back up they would still need to search for others to be able to steal anything from me.