Seal a spare key in a plastic bag, then put it in the soil of a potted plant, possibly somewhere between its roots (or bury it somewhere shallow in the lawn).

Seal a spare key in a plastic bag, then put it in the soil of a potted plant, possibly somewhere between its roots (or bury it somewhere shallow in the lawn). Potential burglars are not going to grub through all that dirt while they don't even know there's a key hidden in there. When you lose your key and have to recover the hidden spare, uprooting a potted plant is a small cost to pay ;-) Leaving a door key under a potted plant is indeed a bad idea.