I must admit, considering how lax airports seemed to be, back as a crazy teenager I was always tempted to smuggle stoopid stuff just past the sensors to prove a point too, but of course my sense would get the better of me. Plus, reporters used to do it too, every once in a while. However, nothing ever seemed to come of it.
Once though, I had this stone container which was bringing back from overseas, maybe about 25 cm high and 15 wide and fairly thick walls. My guess is that on X-ray it'd be hard, if not impossible, to see inside. I put it through the machine, but I don't anyone was even looking so even if the X-ray machine could penetrate its walls, it would have been irrelevant. Never once was I asked to open it, on the ground or on the plane (too late). This was after one international flight and one domestic flight (Canada).