BTW, Aluminum isn't detected by metal detectors, and while aluminum isn't strong enough to make guns from, if you add a few parts per million of the rare earth element Scandium the alloy is quite strong enough. Smith & Wesson has been making small, concealable revolvers like this for years.
It is stupid to worry about this. 3D printers use materials that can't handle the stress of a bullet being fired. At best, you'd get one shot before it blew up and took your hand with it. Plus, of course, the fact that you can currently make a simple gun without a 3D printer makes this ludicrous.
The only 3D printer I know was used in a previous job to build prototypes for our products. In other words, the printer builds a physical mockup that will resemble the mechanical shape so customers can see first hand how the product is going to look like. Best to my knowledge, a 3D printer may produce a very realistic gun that actually is nothing but a toy.