Airport Screening is very hard.

Extracts from
Why Airport Screeners Sometimes Don't Spot Guns, Knives, Scissors
Sharon Begley for the Wall Street Journal, December 30, 2005
Scientists have long known that the ability to pick out a target in a complex scene suffers when there are loads of things you're not looking for. That may be why screeners at Newark Liberty airport missed a butcher knife in a cluttered handbag this year. In all the visual chaos and "distractors," they missed the knife.

Another well-known security blind spot: trying to identify an item surrounded by distractors that are similar to it, as when trying to find a particular pair of pumps in a stuffed shoe closet.

Security experts [are] most worried [by] "target-target dissimilarity," familiar to anyone who has looked for a beer in the refrigerator. The suds may be front and center, but if the bottle looks different from the one in your mind's eye, it may as well be invisible.

... volunteers learned a number of origami-like targets, the original shape as well as rotated or slightly distorted versions. ... The volunteers got better and better at finding the targets, as real screeners do.

But as soon as they had to spot a target that was slightly different from the one they had learned, performance plummeted.

The volunteers spotted only targets they had seen repeatedly, not variations of them. It was as if someone learned what dogs are by studying dachshunds and poodles, and then didn't recognize a spaniel as a dog.

Just to be sure there wasn't something especially difficult about the origami-like shapes, the scientists had 88 participants try to spot actual knives, scissors and guns in X-ray images of cluttered suitcases.

Again ... people [were] eventually spotting 90% of the contraband. But as soon as slightly different guns, knives and scissors were digitally inserted into the image, scores fell.

Unable to see family resemblances, people missed three times as many novel items as familiar ones.

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