People who have been spies all know: a toothpick wedged in the door crack, a piece of paper clipped in the wardrobe, hair pressed in the drawer, coins placed on the windowsill, thin lines tied with zippers, transparent tape on door handles, fine dust sprinkled on keyboards, paper strips clipped between book pages, potato chips hidden in the carpet, remember the angle of the toilet lid, paper clips hidden in curtain folds, water stains left in glass corners. Many times, a camera is not necessary.