The security personnel miss the loaded handgun and bomb parts at least in part because they don’t encounter them frequently. The security person is working for hours at a time, watching people, and looking at the scanner screen. An expectation develops about how frequently certain violations occur. For example, he or she probably encounters too large containers of shampoo, or nail scissors fairly often, and so expects to see those, and then notices them when they appear. On the other hand, he or she probably does not encounter loaded handguns or bomb parts very often. Bellenkes (1997) conducted research these frequency expectations, and found that people create a mental model about how frequently an event is likely to occur. Unconsciously, that expectation affects how much they look for an event to occur, which affects how much attention they pay to looking for the event.

https://www.blog.theteamw.com/2011/02/10/100-things-you-should-know-about-people-58-people-see-what-they-expect-to-see/