The Thieves' Code is a side task in Kingdom Come: Deliverance 2 that continues your work with the Kuttenberg Guild. To advance, you must uncover the meaning of the “thieves' cant” symbols marked on various buildings throughout the city.
Starting the Task
Speak with Chenyek at the Hole in the Wall tavern after completing 
Feast for the Poor. He wants to learn about the old symbols used by a former local gang.
Speak to Knuckles before leaving. He will ask you to bring the information to him exclusively to undermine Chenyek.
Step 1: Hynek's House
Travel to the Kolin Gate at the northern end of Kuttenberg. Hynek's house is located directly beside the gate.
Inside the room on the left, you will find a man named Hog standing over Hynek's corpse.
The investigation
Talk to Hog to learn that he accidentally killed Hynek in a dispute. Search Hynek's body to retrieve Hynek's Letter.
In the room on the opposite side behind a locked door, there is an Easy locked chest containing the Cellar Door Key. Taking this will make the next phase much easier.
Step 2: Solve the Riddle
Read the letter in your inventory. It contains a riddle about a “gushing fountain” and a “vineyard.”
The “vineyard” refers to Vintners' Street, and the water refers to the central fountain in Kuttenberg.
The riddle mentions “shortly after the midday bell.” If you stand by the fountain at noon, its shadow points directly toward the Trader shop to the north.
Step 3: The Secret Hideout
Enter the Trader shop north of the fountain. If you're getting caught, go from the back: use the small metal door three houses left of the trader shop to access a side alley, then use ladders and parkour to reach the trader's backyard and ultimately his shop.
Head down into the basement. You will encounter a Hard locked door which Henry will comment on. You can pick the lock or use the Cellar Door Key you found at Hynek's house.
Inside the room, unlock the Easy chest to find the Parchment with Drawings. This document contains the cipher for all the thieves' symbols.
Conclusion and Rewards
Return to the Hole in the Wall tavern to deliver the cipher.
First go to Knuckles: earn 150 Groschen.
Then go to Chenyek: earn 100 Groschen and progress the guild questline.
You can allow both men to copy the symbols. To continue the main questline for the guild, you must let Chenyek see the parchment.
Questline progression: Completing this task unlocks the next task: Beyond the Grave (see full dual guide with Thou art but dust…).
The Thieves' Code FAQ
Is the cellar key worth grabbing?
Yes. It skips the Hard lock on the trader basement door.
When do I check the fountain?
About noon so the shadow lines up toward the trader.
Can I side only with Knuckles?
You get his pay first, but Chenyek must still see the parchment to keep the guild story moving.
Total Groschen from turn-ins?
250 Groschen if you visit both Knuckles and Chenyek as described.
What if Feast for the Poor is incomplete?
Chenyek does not offer this task until Feast for the Poor is done.