I have just 3 points.
I know I am getting old, but it seems like just last week there were no commercial 3D printers, they were all home built. Then the hobby reached critical mass, and now there is a big market with lots of commercial models, and people making a living running giant print farms. But it was still very recently that most printers were not commercial. All that knowledge has not had time to go away, the firmware running these printers is still actively developed and freely available. So my first point is you don't need to buy a printer to own one.
My second point is that machine components in isolation are very seldom indicative of the application of the whole assembly. For some sort of AI agent to actually identify the real gun parts among the other non gun machine components would require the AI agent to have access to all the components printed on all the printers owned by an entity. Basically it would require a massive database of everything you ever printed. Logistically difficult and legally contentious, but also there are people and companies prototyping devices which are company secrets on 3D printers. They are not going to accept printers that give all their development designs to the government, and wow, talk about an industrial espionage target.
And finally this whole discussion feels very much like a straw man to me. 3D printed guns just are not that good. You still have to have metal components to make a gun. There is not a plastic printable on your general purpose 3D printer that can come close to making a chamber strong enough to hold the pressure of even wimpy rounds. .22lr has a SAAMI standard maximum chamber pressure of 24,000
PSI. It is also pretty warm. Sure, I am sure you can print a chamber that will fire a round, but I would not shoot it, because you may have a better than 50% chance of the bullet going out the correct direction, but your chances are no where near high enough for me to put myself in harms way.
So yeah, your theoretical fanatic assassin might be willing to use a plastic gun capable of firing a single round in its working life, but to make a functional reusable gun is going to require metal barrels, chambers, and bolts. So the 3D printed gun as a real danger to society is a myth.
Now a slam fire shotgun made with plumbing pipe with a nail welded in for a firing pin, those are actually functional and not hard to construct with a modicum of mechanical knowledge and tools. Still not safe, but more reliable than anything 100% plastic.