We often open our "Fundamental Body Language", "Paralanguage" and Identifying Concealed Weapons" seminars by pointing out that we often fall victim to "change blindness". If we can become skilled at noticing changes, we won't be surprised much by threats.
These videos demonstrate change blindness. Our seminars provide tools that work against the blindness.
There are many resources that address the topic. One example is the book "The Invisible Gorilla" by authors Christopher Chabris and Daniel Simons.
Test yourself:
http://www.gocognitive.net/demo/change-blindness