HINT: Joseph Pientka
Yeah... and...?
Yep. He took the notes during Flynn's interrogation, and he also took the notes of Bruce Ohr's FBI briefings regarding his meetings with GPS Fusion. It's highly speculated that one of the reasons Pientka has been kept hidden is that his FD-302 interview forms of Ohr's briefings differ greatly from Ohr's own notes taken during his GPS meetings.
Why FBI Special Agent Joseph Pientka is the DOJ's Invisible Man