Fool Catcher

A "fool catcher" is any item or device used to give away the unwanted entrance to a place or to go into an area which is restricted or be used when not authorized. In "Thunderball", James Bond puts a hair across the bottom of his hotel room closet to see if the door gets opened while he is away. Other fool catchers are bottles tied together with fishing line on a staircase, a bell or a can of nails placed on a windowsill near the catch which will fall and make noise if the window if opened or greasing the handle of a questionable knife - if it later shows a print, it was used; if it's clean later, it was used.