This attack does not tend to be effective at high-traffic access points where multiple parties are pressing a wide range of digits, however for something like an alarm system keypad which is perhaps operated only once or twice per day, an insider attack may have particular success.
After application of the powder to the keypad, once you suspect that a legitimate user may have pressed specific keys, capture a photograph or take a glance at the keypad while illuminating it with the included UV micro light.
Keys that have been pressed may appear to glow less than keys which are not part of the code in use.
Kit includes a tin of UV-reactive powder, a compact collapsible applicator duster, and a keychain UV light.
NOTE - to reduce the chance of forensic detection, wiping the keypad clean with a damp cloth or sponge after you learn the PIN may be wise, so as to reduce the remnants of powder that was applied during the attack.