This is interesting demo of how residual heat in a safe keypad could expose the key sequence. Using a hand-held thermal imaging device (not cheap) you can read which buttons on a keypad were pressed 5 to 10 minutes after the event from up to 10 meters away. Even though each key press is momentary there is enough heat transfer to distinguish the button in a thermal image. The sequence can be determined since the first button pressed has the least heat. This method break s when buttons are reused in the sequence, but being able to see the unused keys cuts down the number of possible permutations.
[thanks Mr. Mistoffolees]