How about storing spare keys in an unused power outlet ?

OK you may think that someone could find out if they plug something in it and find no power. But what I mean is that you have two power outelts side by side in your backyard or tool shed ..etc . One with real 110v and the other one empty and covered (looks like a spare outlet if you need to wire it in the future). Inside, there is no wiring, only the keys.

Some folks have tried it and they are still lucky no one found out.