During the second sniper section (Level 10 I think) just above and to the left where the limo car pulls up you will see a billboard with what looks like a pair of glasses with targets in them.
Shoot them both and the billboard will fall down to reveal a secret poster. This is also needed for the find all secrets achievement.
Skip to 0:23 in the video (I was checking for swag on this play through and was searching everywhere!)
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