[hakfoo]

Many of them are listed as, say "PMG VF25", meaning they've been through a third party grading program and are in a specific, easily identifiable type of holder.

I'd expect if you tried to take such notes to a high-end dealer or auction house, they'd check a list like this.

In the coin collecting universe, similar encapsulation is commonplace for rare or commonly counterfeited types. Some fraud has had its cover blown because they tried to fake the capsule (for example, to turn a $100 coin into a $1000 one by bluffing on the grade). The capsule serial number doesn't match the record on the third-party grading service's website, or the same number was used on fake holders, so the grading service tagged it as such and re-certified the "legitimate" original coin.