I have a code lock on my tool shed and a house key well hidden inside there. Just recently I asked a good friend and neighbor to go inside and check out a fire alarm in our house. Worked like a charm.

We're a group of close friends that live fairly close together. We're probably going to implement that we don't stash our own keys in some hiding place on our property - we stash EACH OTHERS keys. Friend A has the key to friend B, friend B has the keys to friend C and so on. Any burglar who finds a set of keys in any of these hiding places will be though out of luck trying to find the house it fits to.

Of course the hiding places will be hard to find - but let's be realistic, there are not that many possible options (dry, out of sight but possible to reach etc) and a thorough search by anyone who's not totally dim witted will probably turn up some keys.