This secret is located on the 2nd part of the mission when you are traversing the trees with the red Ninjas. At the start of the 2nd part of the mission drop down and left , kill the red ninja then move forward. At the very next opportunity fall down and left again and you will find the wall you can destroy. In the video I found it the long way , then killed myself and showed the quickest route. Destroy the wall for the secret and also unlock the Tcheco Achievement.