When people do the same thing for long periods of time, like security screening at the airport, they develop an expectation about how frequently certain events occur. Such as how often nail clippers or too large bottles of lotion show up. Because these things happen frequently, they expect them and are on the lookout for them.

However, since they don’t frequently encounter handguns or explosives, they usually don’t notice them. Although the government won’t release official figures of undetected weapons carried through security as part of testing, it’s estimated that 70% of them make it though. People create a mental model of how often something will occur, and they set their attention to that mental model.