In short, virtually all keypad entry systems - as used in various applications, including building access control, electronic lock safes, ATM input, etc - are susceptible to a trivial low-profile passphrase snooping scheme. This attack enables the attacker to quickly and unobtrusively recover previously entered passphrases with a high degree of success. This is in contrast to previously documented methods of keypad snooping; these methods were in general either highly intrusive - required close presence or installation of specialized hardware - or difficult to carry out and not very reliable (e.g., examining deposited fingerprints)...
http://lcamtuf.coredump.cx/tsafe/