On this page of our guide to Uncharted 2: Among Thieves you will find a detailed description of secrets and collectibles available in Chapter 1 - The City's Secret. Our step-by-step walkthrough explains how to find all (2) secrets hidden in the eighth chapter of the main campaign.
The first secret is at the very beginning of this level. After taking control of the hero, turn 90 degrees to the right and go through the passageway (shown in the picture). Then go to the end of the corridor and climb the wall to the upper floor (where the stairs used to be). You will find the item you are looking for.
You'll come across the second secret just before the end of the chapter. It will be almost impossible to miss it. When you fall into a spiked trap on the ceiling, look around the floor (preferably after the mechanism is immobilized). On one of the rocks (not in the water), you will find a treasure you are looking for.