That “Website of the Weird”, BoingBoing, today gives us a classic example of why X-ray alone doesn’t work:

couple of months after 9-11, he got a package through that had a battery pack in the center, with sensors connected by coiled wires going out to all sides of the box. He called the Burbank police, and they called the bomb squad.
They evacuated the building and sent a guy in wearing one of those big bomb shield suits. He had a tiny video camera with him, and he poked a hole in the box away from the sensors and peeked inside with the camera. He came out of the building laughing, holding the box under his arm. He handed it to my friend who discovered it, and told him to open it.

Go see the link for the punch line.