Getoutofmygalley is right, these are humans and they will make mistakes, we all do. That said the people on security have as long as they like to look at your bags, these images are queued and a staff member can look at them as long as need be, it's not a case of them only having the three seconds it takes to pass through the machine. Also, security staff are rotated between the various tasks (search, image check, etc) in fairly short time to keep their awareness at a fairly high level....