For other systems throwing some fine dust or flour over the pad before someone uses it will get you 80% there. If it's dry and someone's already used it recently you can spot what the numbers are by brushing the dust off.
along the same lines, older keypads on things like garage door openers that people rarely change the password for but use frequently have serious wear on the keys that are used in the pword. its very easy to open those up. less obvious than a powdery keypad.
