[svandamme]

if it's just one key and only just in case (not to be regulary used),
simply stick it in the grass/ground somewhere in your garden

it won't run away on it's own
nobody will know it's there, or accidentally find it

and even if they do know it's "somewhere in the garden"
good luck to them on finding it

just make sure it's in the grass, not sticking out , or the magpies will get it