Earn this by not triggering the filled-red lightning bolt icon above enemies' heads.
Black, white, and partially red icons will not prevent this achievement from being earned.
Knocking enemies unconscious or assassinating them will not count as "being spotted".
Enemies finding dead or unconscious colleagues will not prevent this achievement (though it
will alert them).
Quick-Save a bunch at breaks in the action. If you are spotted, do a Quick-Load as this will not prevent the achievement.
Not sure if the enemy spotted you or was merely alerted? Press
, select Stats, press
for "this playthrough" menu to ensure the Number of Times Spotted is still zero.
The key is patience. All of the guards have a pattern. Stay crouched the whole time and learn the patterns and you'll get to the end.
I earned this at the conclusion of Mission 1.