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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.
Is this true even if you've not alerted them? I remember sneaking around in NV head-shot-ing everything with a silenced sniper rifle, and if they weren't aware of me it'd kill instantly