So far, former FBI Director James Comey has twice remarked on specific topics that he cannot discuss in the ongoing hearing about his strange interactions with President Trump.
One of them pertains to Attorney General Jeff Sessions. Of Sessions, he said that officials at the FBI and Justice Department fully expected Sessions to recuse himself from the Russia investigation even before he did so. Asked why, he cited "facts I cannot discuss in an open setting."
At another point, Comey was asked about the explosive and partially falsified dossier that an opposition researcher and former British intel officer (Christopher Steele) had pieced together about Trump. This is the now-infamous document that BuzzFeed published around the time of Trump's inauguration, containing some extremely salacious unverified material.
Senate Intelligence Chairman Richard Burr, R-N.C., asked, "At the time of your departure from the FBI, was the FBI able to confirm any criminal allegations contained in the Steele document?"
"Mr. Chairman, I don't think that's a question I can answer in an open setting because it goes into the details of the investigation," said Comey.
In both cases, these (perhaps necessarily) evasive responses hint that there's more to the stories than meets the eye. They don't necessarily demonstrate anything, but these are both directions that Special Counsel Robert Mueller's investigation could potentially or might already be taking.