The hole will expand to accept any key… but any key besides the correct key will be promptly eaten, the keyhole transforming into a mouth and pulling the key into itself. Visible chewing action and the sound of shearing metal indicate the fate of the key. When finished with the key, the box will give some exclamation of appreciation such as, "That one was tasty!" "There's nothing like a bronze-platinum alloy to hit the spot!" or just a simple belch.
What could be so precious as to be placed in a box that difficult to open… and where could the correct key be?!