My wife and I are on the road fairly extensively. We both carry keys to our fiver, and I have always thought that was enough protection from getting locked out.
This past week I helped a fellow traveler break back into his unit, because a fluke in his lock system put both he and his wife outside and both their keys inside. (Don't ask me how he did this -- I still don't understand it myself.)
But the episode impressed on me the need to have a spare set of keys stored securely outside the unit. I have my solution well in hand. But i was wondering if anyone equally paranoid felt the same need, and if so, how did you solve it?