A couple of odds and ends that I don't have a firm date on, but that happened around now...
As you may recall, after the arrests at the Watergate, Howard Hunt went back to his office in the White House -- he still had access -- and deposited a variety of Watergate and Plumbers materials into it. The safe was then opened by John Dean, who turned over what he thought was safe to the FBI investigators -- and then put some of the materials into a sealed envelope and gave it to acting FBI Director L. Patrick Gray.
Gray took the materials to his home in Connecticut, and around this time he decided to destroy it all.
Meanwhile, Dean had held on to Hunt's incriminating notebooks, and Dean destroyed them, too, at about this time -- after Hunt's lawyers had let prosecutors know about them, at which point the prosecutors realized that they hadn't been given all the evidence. Dean told the prosecutors that national security materials had been given to Gray, who denied receiving any of it.
The other event during this general time period happens because Nixon decides to replace Richard Helms at the CIA (he became ambassador to Iran). Helms, clearing out his files, took the photo of the Plumbers break-in at Daniel Ellsberg's psychiatrist's office and gave them to the Watergate prosecutors, who had no idea what they were -- but did identify Gordon Liddy in one of them.
The cover-up, which had held so well through the election, was starting to cave in on multiple fronts. With the first criminal trial scheduled for the new year, the pressure was only increasing on everyone. And there was still no backup plan in case any part of their story collapsed.