Polymers can be detected by x-rays however a polymer firearm would not necessarily look like what a security guard would expect a firearm to look like and so could go unnoticed.
Real question is would a polymer firearm really work?
A true practical polymer firearm (one in which all of the parts are polymer) would have to be capable of firing repeatedly; as the article stated they had found a magazine; a practical firearm would need a magazine.
The real issues with the printed firearm are springs, barrels and firing pins. Plastics are not very good materials for these parts. To my knowledge you will not find plastic used for these parts in commercially manufactured firearms.
Barrels are subjected to extreme pressures when a cartridge is fired; any currently available plastic would fail if a barrel manufactured in a 3D printer had a factory loaded cartridge fired in that barrel. It may be possible to hand load a cartridge to a low enough pressure for the barrel to survive firing the cartridge but the heat of gases generated by firing would damage the plastic barrel (melt the plastic) enough that firing a second cartridge would be unsafe.
Firing pins are a problem because of hardness. Any plastic hard enough to fire a primer is too brittle to survive repeated strikes on a primer.
Springs suffer similar issues as firing pins. If the plastic is resilient enough to function as the spring as it is required to be in the firearm application it is too brittle to survive repeated cycle in the firearm.
Current materials available to 3D printing are lacking to make these vital components for a practical firearm. I believe that I read that a single use firearm has been made on a 3D printer; fire a single round and throw it away. But I would not want to fire one.
I believe that most any firearm could be printed on a 3D printer with the exception of the parts mentioned and that is a big deal.
There are 3-D printers that can directly print metal parts. These parts are used as anything from gears to jet engine parts.
It is quite possible to print springs, firing pins and barrels.
Governments want to utterly squash and destroy personal printing well before metal printers are priced for the home/hobbyist market.
A citizen who isn't utterly dependent on government and its Bestest Friend Forever, Big Business for every necessity of life is a terrifying prospect for some.