Trivia: The person who introduces Leonardo DiCaprio onstage at the end of the film is the real Jordan Belfort.
Trivia: The humming chant Matthew McConaughey does and Leonardo DiCaprio adopts is actually something McConaughey does before scenes to relax himself and clear his head. He did it before shooting the scene with DiCaprio in the restaurant, and after a few takes DiCaprio suggested that it was put in the scene, so it stayed.
Trivia: It was Margot Robbie's idea to appear nude in the doorway. She was scripted to be wearing a slinky robe, but Robbie felt that her character would not wear a robe, reasoning that her character's body was her "form of currency" and that she'd exploit that to "lay her cards on the table."
Chosen answer: As seen from Donne's reaction when the FBI storm the building shortly after, It should be assumed that Donnie told the FBI himself, and kept the note as evidence. One of the reasons for this could be that Donnie must have felt betrayed or something similar.
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