A paper presented by USENIX Security Symposium explains how PINs could be identified using thermal imaging cameras.
The paper, "Heat of the Moment: Characterizing the Efﬁcacy of Thermal Camera-Based Attacks," showed how an ATM customer’s pressing down keys of a personal code number gives these cameras the ability to catch the numbers from residual heat left behind from the user’s fingertips.
The team at the University of California, San Diego, found that their thermal imaging cameras identified the PIN entered on a keypad more than 80 percent of the time if the camera was used immediately. If used a minute later, it identified the digits about 50 percent of the time. After 90 seconds, the chance of identified the digits reduced to about 20 percent. This was tested using custom software that they wrote to automate their analysis.
To read more about this, visit the published paper here.