In this page you can find all secrets in Uncharted: Lost Legacy that can be unlocked in the first chapter, "The Insurgency". Getting this treasure isn't difficult but, unlike the ones from the prologue, this one can be missed if you won't look around enough.
Treasure 1/1 -Sterling Silver Cigarette Case
The treasure can be found after reaching the rooftop by using the ladder or the stairs. This takes place during the last part of the stage. When you reach the roof you must ignore another ladder (it leads to the higher part of the roof) and look around the place with laundry. The treasure is hidden in the place presented in the picture (the place is located slightly to the left from the place where you start your exploration of the roof after using the ladder).