SPOT is based on the Facial Action Coding System (FACS), a method developed by Paul Ekman to read faces (familiar to fans of the show Lie to Me): http://www.denverpost.com/editorials/ci_24533526/face-it-tsa...

While the data behind using FACS to spot emotions (EMFACS) is solid (https://books.google.com/books?id=W_npWdrHGFcC&pg=PA47&lpg=P...), the record on using it to spot lies, terrorists, or otherwise discovering the intentions behind those emotions is not good (http://www.washingtonpost.com/local/trafficandcommuting/gao-...).

One huge issue is that a scared terrorist looks the same as a scared person singled out for screening, and there are many times more of the latter. This is a specific issue Ekman brings up in his work when using FACS for lie detection. FACS is much better in finding out how someone is feeling rather than what they're thinking. But you need to know intentions to figure out if someone is a terrorist.