On this page of our guide to Uncharted 2: Among Thieves you will find a detailed description of secrets and collectibles available in Chapter 9 - Path of Light. Our step-by-step walkthrough explains how to find all (3) secrets hidden in the ninth chapter of the main campaign.
After climbing on the first part of the blade, you will walk on a red face and unlock a new passage (during a short cutscene, the camera will show you how the steps extend - use them to go down and reach the first mirror). However, before you do that, go to the end of the balcony and pick up the treasure.
The second secret is located near the third face, on which you have to use a key to unlock a secret passage (the collectible is a few meters earlier, hanged high up).
The last item was hidden in a secret chamber, at the very end of the chapter. You'll get there whether you want it or not. You can find it on the left, behind the pillars.