[snip]
Leaving a key out in the open for someone to find is a bad, bad, bad idea. Do NOT do this, please. Watch "It Takes A Thief" on the Discovery Channel to see just how easy it is for a predator to break into your home, let alone you leaving an invitation to do so under that garden gnome in the side yard.
If a person wants to have a back-up key somewhere, I would propose a few options: (1) leave a key with a couple different neighbors rather than one; this reduces the chances that the one neighbor who has a key isn't home (2) give a key to a friend who lives or works close by (3) I don't know if these are still made, but I used to have one of those credit card-sized plastic emergency keys for my car I'd carry in my wallet. I'd bet they could cut one to a door key. Of course, this only works for a woman if she has her purse but not keys, but most women I know keep their keys in their purse, so...FWIW. (4) get one of those electronic, weather-resistant lock boxes that require the person punch in a code to gain access, and leave a key in there. You can get one for $25-50 at any real estate supply company (Google is your friend!).
Personally, I prefer option 4, and given the number of times I've locked myself out of the house in the past two years, I am asking for one for Christmas!