The real question you should be asking yourself is how hard it is to fake these. If I get hold of someones copies, can I use them as template?
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They were clearly betting on the fact that no one would notice they are there. What scares me is we're just finding this out. How long have criminal organizations and rogue nations known about this and what have they used it for?
There were magazine articles, newspaper articles, and news site discussions about this years ago. They covered it being added to stop color laser printers and dye sublimation printers from being used for currency counterfeiting. That the tech community has this short of a communal memory astounds and saddens me.
Even beyond the public knowledge of this tactic, that Reality Winner was working at an intelligence agency and was silly enough to think said intelligence agency couldn't track what had been printed in its own offices is laughable. Either she had no business working in that environment as she clearly doesn't understand their mission and methods or she's a scapegoat.
* 2014 - PC World - http://www.pcworld.com/article/229647/counterfeit_money_on_c...
* 2004 - PC World - http://www.pcworld.com/article/118664/article.html
* 2005 - Washington Post, stating it had been in use at least ten years, and that at least one version of the yellow dot code had been broken. - http://www.washingtonpost.com/wp-dyn/content/article/2005/10...
* 2004 - Slashdot - https://hardware.slashdot.org/story/04/02/06/1513255/hp-disc...
* 2004 - Geek.com - https://www.geek.com/news/color-laser-printers-allow-feds-to...
I could probably easily find more.