When I first moved to Japan in October of 2003 (Wow, has it really been that long ago!?) we lived off base but got all of our US Mail at the Post Office on base. This was the KEY that I used to check that mailbox for the first year or so. Then it stopped working and the box got re-keyed. I should have turned in this key, but nobody ever asked for it and I kind of liked it too much to offer it up without being asked!
Five years after leaving Japan, the little key that I used for my mailbox in Japan still has a home on my key chain. Along with my base apartment key (Oops! Though I never really had a chance to return it since my entire last day in Japan was spent at the British Embassy getting my visa!), and the replacement mail box key. I'm probably on some sort of watch list on the base in Yokosuka now, since technically I've "stolen" three keys. But is it really stealing if they never asked for them back? Okay, okay, so it probably is but thinking this way allows me to sleep at night!! ;-)