This achievement is earned for finding the fifth secret and can be found on Checkpoint 16
It can also be found in this video, with a text description of its location below
[3:35] - Secret 5 of 13
Upon reaching the rooftops, you'll come to a slide down the roof of a building and then a small jump across a gap onto a pipe. Instead of going up, go down, and jump across to the ladder. Head down the ladder to find the secret.