Simply leave any magnet (key) sitting on the shelf. You could leave any number of magnets on any number of shelves. You would still have to know where to place the magnet to open the hidden lock. To the average scum-bag, it is just a magnet sitting on a shelf.
I would consider hidden magnetic locks over simple magnetic closers simply because I do not want the hidden compartment opening inadvertently - when moving a piece of furniture, or bumping into a spring loaded magnetic door.

