On October 19th 1972 Haldeman told Nixon that the main leaker to the Washington Post was Felt who was at the time the number 2 man at the FBI. Haldeman did not guess. According to the Nixon tapes Haldeman stated it as fact and Nixon accepted it as fact. The tape clearly shows Haldeman naming Felt as the leaker. Nixon did not question the statement at all. There was no "How do you know?" Or "Are you certain?" question from Nixon. Nixon proceeded as if what Haldeman said about Felt was a known fact... a certainty.. Nixon acted as if it were a fact not a conclusion drawn from analysis.
How do you suppose Haldeman knew with certainty that Felt was the person leaking. From what was published in the Post it was clear that someone was leaking the results of the FBI investigation of Watergate. The members of the FBI investigating team knew every thing that Felt knew. Felt only knew what that team was reporting to him. How could Haldeman know that Felt was the source and not some other member of the investigating team? How could Nixon instantly assume that Haldeman was right?
Both Nixon and Haldeman has to know something that was not on the tape.. They both had to know how Haldeman knew the leaker was Felt!!
There are only two posibilities.. The Nixon administration had a mole or bug in the FBI or they had a bug or mole inside the the Washington Post or both.