Well, since most of the gun is going to be made of plastic, and the only metal component is likely to be the bullets and firing pin, it stands to reason the only way the gun itself will be detected is in a physical search. This is a serious problem, because whatever you think of the government or gun rights, there are people who should not have guns, and unregulated 3D printing makes it all too easy for existing safeguards to be circumvented. The only silver lining is that right now 3D printing isn't all that great for guns at the moment (they tend to be useful for only a couple shots).
I don't think that normal people will be doing wrong with this tech, but it probably gives a tool to the bad guys to fabricate parts and then get it passed through scanners. I sort of agree with the bulletin's point and I believe they are worried about the bad guys using this at public places and not regular people trying out in their backyards.
Fearmongering. While the gun may be undetectable in certain situations (But seriously, not really), Bullets with their blatantly brass or steel cases, copper plated steel or lead projectiles, and incredibly explosive powder will NEVER make it past the same checkpoints.
And anyone without a Felony criminal record can legally manufacture guns in the United States, whether it's made out of plastic, steel, or space rock. All you have to do is fill out a short amount of paperwork and pay a small licensing fee.
Most people don't know this, there are no laws that prevent a person from making their own gun. There are laws against buying certain guns. The only way this can acted upon is through violating constitutional rights.
Blocking currency copying is done by integrating non-copyable components in the source and watermarks that get detected to prevent copying.
For 3D printing, you can use your own CAD tools to re-create any design based on blueprints so there is no trace of the source material for the 3D printer to detect and block copying of.
Trying to regulate 3D printed guns and proliferation of blueprints is somewhat ridiculous since a gun is little more than a hollow tube with a holding and firing mechanism attached to one end. A crude gun requires very little knowledge beyond your ammo's dimensions and the minimum thickness the barrel wall needs to be to contain the expanding gas.
I'm glad the DHS at least acknowledged that this is one battle they are extremely unlikely to win due to how little effort this requires once the technology is in people's hands.
"so there are now plastic bullets? as in the entire bullet and casing is made of plastic?"
The point is that small bullets (e.g. .22 LR or 0.38 Special) have about as much metal on them as the zip on your jeans, or your wedding ring, and far less than a belt buckle. Metal detectors are obviously deliberately set to NOT go off for mundane items, thus bullets will NOT be detected by standard metal detectors unless you want them set up so every pair of jeans, every ring, every necklace etc rings the alarm - i.e. for basically EVERYONE that steps through it.