The Antique Pocket Watch is the thirteenth treasure that can be found in Uncharted 2: Among Thieves.
The Antique Pocket Watch is found in Chapter 2 - Breaking and Entering. After you managed to climb up to the window, and after you navigated across the steel bars underneath the metal balcony, you'll be forced to shoot an enemy below with one of your tranquilizer darts. With that done, you can jump down to the platform he was once occupying and move forward. Jump over the balcony's edge to the copper roof below, and follow the edge of that roof to the right, then left around the building. Look behind you and to the left at the building you just circled. On the wall, alongside a huge tapestry of some sort, you'll see the telltale flash of a treasure. Shoot it down from its perch, and then jump down to the area below to grab it once it's fallen.