Insanity that usually doesn't manifest itself visibly, and usually is never found. Includes things like stalking, planning (but not enacting) murders, etc.
Guy 1: Did you hear what happened to Blake yesterday?
Guy 2: No, what happened?
Guy 1: He got arrested! They say that he could have snapped at any moment and killed someone! They found lots of creepy things in his house.
Guy 2: Sounds like a case of domestic insanity.
Guy 1: Definitely.