I've long wondered this too because of a couple things one in research and one that President Trump kept saying. He said 13 Angry Democrats.  So Mueller stacked basically some of the most partisan and abusive prosecutors on this special counsel.  It would be the last thing anyone would expect for anything against a Democrat and most especially Clinton or Obama to come out of that. However what I read about Mueller is he tends to set people to work on other things while he keeps his own work close and talks to no one about it.  I wish I could remember who said this but it was a long time associate ago waxed on about Mueller's goodness and yada yada (whatever seeing his track record) but his only complaint working with him was he wouldn't delegate and kept most to himself. Maybe there js a reason Weissman and Rhee were kept busy with Manafort, Gates and Russian troll farms and a ham sandwich then Corsi, Caputo and Stone.  Maybe Mueller had his own project going with info from Flynn and his own spider web of evidence he has collected for decades.