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That is true, a good distance away and moving to a new spot after you did a headshot helped.
To make you undetectable, it's best to walk slowly (cap lock) which result in the Caution bar slowly disappearing instead of increasing toward Danger;
If the bracket start moving closer to the word, it mean the chance of being detected increased.
Bracket moving away from the word - less likely to be detected then turn to Hidden.
Welcome to Fallout.
One of the 'features' of the game is that visually unarmored enemies will still need several direct hits with high caliber weapons before they drop.
It's bit irritating hitting humans in the head with a ridiculously highly leveled sniper rifle bullet - or even a missile - and have them shake it off like I'd hit them with a tennis ball.