This side mission can be triggered after you've broken into a books store at the end of this chapter (before using the rotary doors). Go to the lower level of the books store to enter a reading room and locate Voxophone #56. Pick up this item to start the quest.
Return to The Salty Oyster bar located in one of the previous halls of the station. Break into the bar and go behind the counter. You'll find a golden button here and pressing it will open a secret door.
The hidden room located behind the bar contains tons of valuable items. Most objects can be found on the main table - a bag with money, a locksmith's set, Infusion #20 and a new vigor - Return to Sender. Also, make sure to check the corners of the room. There's a steel basin on the floor and it contains Voxophone #57.