On this page of our guide to Uncharted 2: Among Thieves you will find a detailed description of secrets and collectibles available in Chapter 20 - Cat and Mouse. Our step-by-step walkthrough explains how to find all (4) secrets hidden in the 20th chapter of the main campaign.
You can find the first treasure by the place where you will be attacked by the first opponents that are standing on the roof. It's near the spot where you will have to jump over the wall (even before doing so).
The second secret can be found during a dangerous moment - after jumping over the rod and a wall you will fall to the ground and almost get smashed by a tank. The treasure is located in the area where this event takes place. It is just before the position of the first two attackers (one of them will have a shotgun).
The third treasure can be found shortly after you descend from a narrow rocky ledge, which is located above the chasm (a tank will appear above, partially protruding over a cliff). The treasure is hidden on the roof of the building which you're going to jump on, near the colored flag.
The fourth treasure is really hard to find. You'll find it at the end of the chapter, about halfway through the process of destroying the tank (and taking away the enemy's rocket launchers). This item is hidden in a spot that can be reached by jumping through a small hatch in the roof. You can find it on the adjacent roof to the one where you will obtain the second rocket launcher (the treasure is under the passage).