@bythewell - An X-Ray might show up something like that, but generally those body scanners they have at many airports are next to useless because it's too easy to fool them. Unless they are willing to do a full cavity search on everyone, there isn't a fool-proof way of making sure that people are safe.
But those scanners tend to make people feel like they have done enough, which is dangerous. I read an article the other day by a guy who reckoned he went through one of those scanners wearing shoes with rubber soles that should show up on those scanners in the same way as plastic explosives would. He was an airport security person himself, so he knew how they worked.
But he wasn't stopped and was allowed to get on the plane. I mean, with all the money and time spent on this kind of security, you'd think we'd at least be safer, but it seems like the opposite is true.