Whenever a guy goes postal, nobody who knew him in any capacity ever cops to noticing anything askew about his manner. (Has a woman ever gone on a shooting rampage?) The guy is invariably described as nice, quiet, considerate, well liked, etc. Because admitting to having noticed even the faintest sense of turbulence or unrest in a shooter-to-be would mean that the observer might be regarded as being very slightly responsible for the carnage. It’s safer to say you detected nothing.