There is only one gun that does not show up in a metal detector (a ceramic Glock), and that will show up on X-Ray so has to be carried on the person, so will easily be found on a search. It is an extremely expensive gun which is difficult to acquire. As far as I am aware it has never been implicated in a hijacking.
Could you point to a reference to this mythical gun?
I have two Glocks (Glock 17 -- 9mm, and Glock 23 -- .40 S&W). There's no ceramic in them. The frame itself is polymer (aka plastic), but the action parts within the frame are steel, as are the rails which hold the slide. The slide is steel, as is recoil spring. The guide rod is plastic. The magazines are plastic, but have steel liners. The barrel is, of course, steel as well. The cartridges, of course, are made up of brass cases and lead projectiles (usually jacketed with copper).
The plastic used in the Glock frame is dense enough that it shows up quite well in X-rays. And the steel in the slide, barrel, etc. would set off the metal detector.
While I've heard people claim that there are undetectable ceramic guns, no one has ever coughed up any evidence of one and they certainly aren't for sale here in the states. I've come to believe that they exist only in the Tom Clancy novels and the like.
And, of course, Glock is not the only manufacturer to make handguns with plastic frames, though they may well have been the first. Heckler-Koch, Smith & Wesson, Walther, Kahr, etc. all offer plastic-framed guns.
OFBSLF