Why would a 4 star general or the CIA Director use gmail for sexting anyway? If he is so darn smart and all? Felt he was above everyone else, just like his boss the 0?
So, how do you get rid of cabinet level officers? Well, you build a case that shows there's some sort of illegal activity associated with their office, or closest associates. That gets the federal cops going, and they can subpoena stuff, and tap government phones or computers, and there you have it.
A Postmaster General once got in trouble for some insider trading and manipulation involving the Cape Hattaras Stamp. He left after agreeing to give back the stamps.
I"m assuming this is more common than 2 cases in 40 years, but most of the time the cabinet level officer agrees to giving up the loot, and the cops turn him loose with a warning. Whistle blowers find this behavior is not good for their careers BTW