Overview
Hidden Messages of the Nilfgaardian Kind is a secondary quest in The Witcher 3: Wild Hunt, set in Novigrad. It becomes available a couple of days after the Novigrad part of Never Trust Children is over. The start is easy to miss, because no marker appears.
The dead drops
On the road west of the Crooked House, near the archway before the morgue, two shady figures, one keeping watch and one crouching, finish their business and leave. Witcher Senses reveal a loose brick in the archway with a spy’s note behind it. It points to a second drop near Crippled Kate’s, in the archway just east of the brothel. That one holds another message and a key.
The sewer
The final location is under the Gildorf district. Open the grate by the well and head down. Three drowned dead attack, one from behind. At the dead end lie the bodies of the people who came to make the exchange. The locked chest holds the last note, coin and some loot.