So what do you think is it possible for someone to board to a flight in Zagreb with a loaded handgun in his backpacker? And made it through security without a problem. Imagine to have someone siting next to you with loaded gun in his hand baggage, have small talk, share a cookie…

I find this example in a book “100 things every designer needs to know about people” :

A Mental model about frequency

Security personnel miss the loaded handgun and bomb parts at least in part because they don’t encounter them frequently. Security develops an expectation about how frequently certain violations occur. For example, he has probably encountered containers of bottles or nail clippers, and so expects to see those, and then looks for them. On the other hand, he probably does not encounter loaded handguns or bomb part very often.

He creates a mental model about how frequent any of these items will occur, and then, unconsciously, starts paying attention accordingly.