This achievement is earned for finding the third secret and can be found on Checkpoint 9
It can also be found in this video, with a text description of its location below
[1:49] - Secret 3 of 13
Jump down from the top of the farmhouse into the wagon of dead pigs. Take the wagon and pull it to the left, next the building. Get on top, pull a plank off the side of the building and go inside. Use the rope to swing to the left and grab the secret.