BioShock Infinite: Soldier's Field Find the Cipher
In the restroom of a bar, you find a code written in blood. A soldier lays dead near the code, and he has a picture of a cannon with a circle on it:
Elizabeth can decrypt Ciphers that hide secret Vox Populi compartments. Find the Cipher and its corresponding code book to uncover hidden treasure.
 Code Book Location Clue
The pic of the cannon is the clue, and the red circle around the tip tells you that the Code Book will be found there
Go outside and to the building across that says "Tickets":
Code Book Location: Look in the hole of the cannon to find the codebook:
Elizabeth will decipher the code as "Tip the Hat to the Vox"
Go to the adjacent bathroom and interact with the hat:
This will open a secret door:
Loot the room as it has lots of stuff in it