On this page of our guide to Uncharted 2: Among Thieves you will find a detailed description of secrets and collectibles available in Chapter 23 - Reunion. Our step-by-step walkthrough explains how to find all (9) secrets hidden in the 23rd chapter of the main campaign.
You will be able to collect the first treasure in the starting location (at the top). All you have to do is use the first rope you encounter and before you jump on the footbridge leading to the window, you have to swing towards one of the yellow pillars. This is the spot where the desired item is hidden.
The second secret is on the battlefield - you will come across a tree growing on a platform (very characteristic, you can't miss it). There, you will be attacked by two enemies equipped with heavy machine guns. To get to the top of the arena, you'll have to climb long stairs. The treasure is hidden on their right side, on a partially destroyed shelf.
The third secret is in the adjacent 'arena', where you will once again confront two enemies carrying heavy machine guns. You'll find it on the right side (by looking from the direction you came there, towards the exit window which leads further on), in the upper part, near one of the red pillars. To reach the treasure, you'll have to shoot at it first.
The fourth treasure is near the third one. To leave the arena, you'll have to go through a window and enter a dark room. The treasure is hidden there, attached to the ceiling (shoot at it so it may fall down to the ground).
When you leave the darkened room, you'll get on the roof. From there, cross over the bricks to reach the upper balcony. Before you move any further (on the red beams), jump on the roof below. The treasure is hidden under the wall of the balcony - shoot at it to knock it over.
Another treasure is nearby. After you have passed through the aforementioned red beams, you will reach a room with a wardrobe. Climb it to get on the roof (the one in the picture). A small sculpture will be attached at its upper, outer end. Get close to it by hanging on the edge and collect the treasure.
Once you have solved the puzzle in the tower, you will take a secret passage to the roof and then head to the remote building by using a suspended rope. You will get to a darkened, narrow passage. Follow the path until you reach ruined stairs. Jump over them and turn around. That's how you'll notice the item you're looking for. It will be hanging under the ceiling (shoot at it and collect the treasure).
The penultimate treasure is at the top of a sculpture, near the secret passage (which will turn out to be only a mechanism later). Knock it over with an accurate gun shot, and then climb on the back of the statue to pick it up.
The final treasure of this chapter is hidden in the location where the last battle takes place. After the secret passage is revealed, use the zip line to descend between the buildings. Then, jump down to the ground. Under one of the roofs, near a closed door, you will find the item you are looking for.