During the Echoes mission, you'll reach some stairs leading up to the outside/above ground and will hear a howl from the Watchmen. Simply stay down on the stairs (or go explore the tunnel as I do in the video) until the achievement unlocks.
Check out the video below for a full playthrough of the game showing all collectibles, miscellaneous achievements like this one, and the Moral Points I obtained (I skipped a lot of them) in order to get the Redemption achievement. The Echoes mission starts at timestamp
25:34.