Velvet Assassin

While on your way though the last part of the level heading to the bell tower you will go up a spiral staircase. At the top will be 2 guards, one facing away from you and another to the right behind some boxes. Kill both guards and at the top of the stairs you just walked up you will see a crate, push the crate out of the way and you will find this collectable hidden in a small gap which the crate was concealing.