To begin the Missing: Nadia lost and found quest in Dead Island 2, interact with the poster on the missing board at the Serling Hotel on Ocean Avenue. We found this after beating the game (might be found sooner). For this quest, travel to the Metro then head east from the fast travel point. You’ll find Nadia’s Map journal on the door.
Next, you’ll have to track down the 4 digits of the code to enter Nadia’s Combination on the Security Keypad. The first is 0. Head back down the hallway going towards the fast travel point to spot the blue 0 painted by a door.
8 is the next number and can be found on the roof in a train car at the location we’re in as shown on the map below.
2 is the next one and it can be found on the wall in a hallway south of the above train car.
6 is the last and can be found on the wall in the room west of the above number.
Once you have all the numbers, return to Nadia’s door. Unfortunately, she didn’t make it and you’ll need to take her out. Once you do, loot the two journal entries in the room she busted out of. You’ll then be able to loot the chest in this room as well. Afterwards, the quest will complete. As a reward, you’ll receive 2,500 XP.