In case anyone ever wonders this again...
I thought the same when I played thru to get this, and I got the Going Dark acheivement as you did.
Turns out I took a bullet during the "breach" and didnt notice or remember. It was the only spot in the level that an enemy even saw me to take a shot, so it had to be the time it happened.
100% - Hitman: Blood Money - Bioshock - The Godfather - Mafia 2 - The Godfather 2 - The Saboteur - Sleeping Dogs - Skylanders Giants - Deus Ex: HR