Hidden Messages of the Nilfgaardian Kind is a secondary quest in The Witcher 3: Wild Hunt.
Walkthrough
To start the quest you need to have completed Never Trust Children first, then in The Bits head north along the road that's to the west of the Crooked house. As you near the archway just before the morgue location, you'll see two shady individuals, one standing guard and the other crouching. As you get close enough they'll finish whatever they were doing and leave and Geralt will make a small remark. Note that this interaction is very easy to miss as no marker shows up in the quest.
Head to the archway they were standing near and use your Witcher Senses to see a brick in the wall and interact with it to get the first Note left by spy. Read it to learn the next drop location is near Crippled Kate's, earning 10 
.
Head to the brothel area where, in the archway just to the east, you'll find the next hidden brick and message, this time with a key. Read it for the final location (earning another 10 ).
Head to the Gildorf district, near the well, and open up the grate to head down into the sewers. As you head toward the search area here, three drowners will appear. Once they're killed off, in the dead end hall you'll find the bodies of those who were trying to make the trade. Loot the chest here for the final notes and read it to complete the quest, earning 25 .
Journal entry
- During his travels through Novigrad the witcher witnessed an extraordinary event. Right before his eyes a mysterious man put something in a hidden dropbox located in the wall of a building. Geralt looked into the box and found a note which directed him to the next location where secret notes were left.
- Geralt followed the thread back to its source and wound up at a place where Nilfgaardian spies loaded and unloaded contraband. Geralt might not take a keen interest in politics, but he noted the emperor certainly seemed to be taking a keen interest in Novigrad.
Objectives
- Read the note.
- Find the next stash using your Witcher Senses.
- Read the note from the dropbox.
- Take the loot from the spies' chest.