dirty keypad provides a tell

This looks like pretty good advice - clean the keypad.

They do this all the time in movies. Someone wants into a room that is secured by a keypad activated lock. They use a black light to see which keys are most used. But in this case, found at NedHardy.com, the dirt gives it away.

In the movies they get the sequence right on the first try. But in reality the uninvited entrant would still have to do some trial and error to get the numbers in the right order. And if the system sends an alarm after too many wrong tries, the potential thief might get to explain himself to the police.

But it's still a good idea to keep it clean.