In the boathouse scene where Michael tells Tom he's going away, and he's gonna be the Don, Tom questions if Rocco or Al could be the mole. Michael explains are their people are businessmen, and they're loyalty is based on that. What a short memory Tom has. He should have remembered that with the Tessio situation. "It's the smart move. Tessio was always smarter". Tessio even tells Tom before his final ride, "Tell Michael is it was only business. I always liked him".