That's an interesting problem and one I have been trying to solve. How can she get the key or whatever without being able to put it back in such a way one wouldn't notice.
If you aren't going to be away that long, I would put the key or knife in a shallow pool of indian ink in a saucer, then make sure the indian ink is not available for her to replenish it. If she tries to be cute the indian ink would stain her long enough for you to know she needs a little punishment. If she really needs to get out who cares about a few black stains around the place.