On this page of our guide to Uncharted 2: Among Thieves you will find a detailed description of secrets and collectibles available in Chapter 5 - Urban Warfare. Our step-by-step walkthrough explains how to find just one secret hidden in the fifth chapter of the main campaign.
The only secret that can be found in chapter five is Strange Relic. You'll find it in the open channel, as shown in the picture. This is the moment of the game when the enemy helicopter appears. If you are not interested in the item, you are not even forced to go down the street. Just continue on your way and don't bother. But when you decide to pick it up, the blue gutter will serve as a way back.