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Bill designs incorporate a lot of public features in order to help people figure out when bills are legit.

I do wonder how many non-visible features they hold back, in order to recognize the case when high-quality fakes have landed.

Does this actually happen, though? I mean, are there examples of hidden features on older currency?

I can't find the link now, but I remember reading an article stating there are three levels to this thing.

One, the obviously visible and known features, such that metallic strip many notes have. Know to both the users and would be counterfeiters.

The know but not obviously features, such as micro text or the things that become visible when seen under those purple lights (UV?).

Secret features only known to a very small select group of people in the money printing business. These are last resort identifiers of fakes.