Most likely it came from North Korea about 6 years ago, somebody used those funds to buy something and it went into circulation, something big and expensive.
These notes are extremely good and had a lot of banks worried some years ago, actually this is old news of a sort.
No chance. The article says that they've detected 19 different versions of the note, each version corresponding to a tiny change in the U.S. engraving plate. Why would any counterfiter go to that trouble when they can keep on cranking out their older version? A more plausible explanation might be that every time the Bureau of Engraving created new plates a second set was made for the purposes of this program. Then they embedded an identifying flaw and kept on printing their notes for their own purposes.