KeenGamer: “Bioshock Infinite is full of hidden secrets and easter eggs galore. Here’s a Bioshock Infinite side-quests guide to help you find them. One of the trickier things to complete are the side-quests scattered throughout the game in the form of hidden chests and Vox Populi secret rooms containing upgrades and goodies.”
Source: N4G PC BioShock Infinite Side-Quests Guide: Every Vox Code and Hidden Chest