The Mongol Paiza Passport is the twentieth treasure that can be found in Uncharted 2: Among Thieves.
The Mongol Paiza Passport is found in Chapter 4 - The Dig. You can either kill all enemies AFTER you enter the cabin which started a cut scene and then the enemy waves begin. Or you can kill the first few enemies that are occupying this area when you arrive (sully will grab a sniper rifle and "help" you out. You will notice that there is a ship wreck around you. Look up at the tallest masts (with your back to the cabin, but not actually entering the cabin) and you will see the glimmering dot at the top of the mast for you to shoot down.