Fingertips, emit oils on the exterior wall-mount keypads when the code is punched in; e.g. 1-4-5-6. Other keys on the pads that are never used become dirty with wind, rain and dust. Using a flashlight with a blue or red lens, or even using angles in daylight hours, you can see where the 1-4-5-6 numbers appear clean and/or shiny with use. A burglar could determine the four most used or clean numbers he could “crack” the code in a few minutes or less.