On this page of our guide to Uncharted 2: Among Thieves you will find a detailed description of secrets and collectibles available in Chapter 4 - The Dig. Our step-by-step walkthrough explains how to find all (3) secrets hidden in the fourth chapter of the main campaign.
The first secret was hidden in the area of the camp, where the largest battle will take place. Start your search after the battle, that is as soon as you blow up the tower. The secret was hidden on a mast, in the central part of the arena - shoot at it and collect the item.
The second secret is in the first room of the ruined temple. It was glued to the wrist of a carved figure hanging from the ceiling. It's located next to the entrance to the hall with a blocked passageway.
The last treasure is also hidden in the same temple (at the end of the level), and you will be able to collect it once you obtain the artifact and get rid of the torch. When the cutscene will be over, turn left and look up towards the breach in the ceiling - a treasure was hidden there. Shoot at it in order to collect the item.