On this page of our guide to Uncharted 2: Among Thieves you will find a detailed description of secrets and collectibles available in Chapter 3 - Borneo. Our step-by-step walkthrough explains how to find all (4) secrets hidden in the third chapter of the main campaign.
The first secret is at the very beginning of the stage, not far from where you start the gameplay. Head for the trees and the swamp, and you'll come across the first cargo. You should see a fallen tree right behind it. Climb up on it and you will get to a parrot and a collectible hanging on a tree (shoot at it to knock it over and collect the item).
The second secret is near a spot where the first two enemies will attack you. Just like the last time, pass over the fallen tree. You will find the secret on a barrel.
It won't be that easy with the third secret. At some point, you will be forced to walk on a fallen tree over a swamp, which you had been crossing a while before (shortly after you pushed down a boulder to unlock your way). Enemies will attack you on the other side. Defeat them and head towards the direction from which they were shooting. You'll come across a wooden wall there. Climb it upwards, but don't jump - instead, while in the upper part, go to the right, and you will discover a hidden passage (as shown by the arrow). Now it is enough to follow the path to the end and pick up the desired item.
The last secret is even more tricky. After a brawl in the lake, climb a rope to reach your companion. Remember that instead of heading up the stairs, you should go to the right and jump over the chasm (as shown by the arrow). Go ahead a few meters and you'll come across the protruding rocks on the left. Use them to go higher and jump towards the tree to reach the wooden rods. By swinging, jump on the thick logs, and from there on the next stone ledge. Jump on another ledge and obtain the last secret.