In Northeast of Rosemary & Thyme in Novigrad in the map, just left if facing the entrance of the Rosemary & Thyme, near the two small stone bridges. Geralt will smell rotting flesh. Follow the scent to a dead body with some crows around. Examine the body and follow the blood trail of his foot steps using your witcher senses to a house just left of the Rosemary & Thyme. Inside the house will be three Level 20 of Cleaver's henchdwarfs. A dialogue will ensue between the witcher and the first two dwarves. Apparently the murdered victim was named "Derrick" and they want a key that Geralt found in the dead body to open a dropbox. From here you can give the key to the bandits for a small reward, or beat them up and loot the treasure for yourself. If you decided to kill them, the treasure chest filled with florens and crowns will be waiting upstairs to loot.
Journal Entry Edit
If Geralt decide to kill the bandits:
Geralt spent some time rutting around in the dank alleyways of Novigrad and at one point came upon the body of a mysterious man. A cursory inspection revealed that this unfortunate soul had been struck several times with a sharp object. Though he seemed to have managed to flee his attacker and hide in this alley, he had bled to death shortly thereafter.
Geralt realized he could follow the trail of blood and find out who had done this to him - and why.
It turned out bandits had murdered the man. The witcher could not restore him life, but he could at least punish the guilty party. Which is what he did.
This quest is only available in night time.