I have a spare housekey hidden near my residence, but not actually on the property. This idea I got from geocaching.
What I did was take a standard metal coathanger, snip & straighten it, hammer one end flat, and form that end into a hook. So, it resembles an upside down J. At the straight end, I taped a set of spare apt keys to it. I then went to a storm drain on my street, up a little ways from my apt, and dropped it in, with the J part of the hook over the grate. It blends in perfectly, and no one EVER looks in the storm drains. I check on it when I go for a jog, and pull it up once a month or so to ensure its still attached. Even if someone finds it, they would have to figure out who it belongs to then which door inside the complex.
you didn't get that idea from MY geocache, did you? i have had a micro hidden in a storm drain like that for a few years...haven't seen it replicated anywhere, though.
i've been meaning to hide a set of keys ala GCing for a while now, but have yet to make the effort - despite having locked myself out at least twice since deciding i need a back-up plan.
dolt.