Count Reuven's Treasure - Novigrad
This quest is automatically given to you upon completing the Broken Flowers Story Quest in Novigrad. First thing you have to do for this quest is use your Witcher Senses to investigate the area. Right where the wall is blown open you'll want to search the pipe hanging from the ceiling and on the other side of the water search the metal bars to update the quest.
Before going any further into the sewers you will want to drink Pops Mold Antidote, that will protect you from the toxic spores in the air. While following the water be sure to use your Witcher Senses too, you'll find some jewels along the way that were dropped. There are also boxes and bags you can loot - one of which will have a new crafting diagram for you.
If you wish use Igni on the corpses you come across in the sewers to stop Drowners from spawning. Once you get to the big room search upstairs for some goodies and then search the compass/map location for the last remaining clue, shown in my screen shot below. Upon finding this clue you can return to Dijkstra and speak with him to update the quest again.
Fill Dijkstra in on what you've found then explore the bath house for some more clues. In one of the bath tubs you'll find oil floating on the surface and the other half of the cylinder on the backside of the tub. Geralt will call Dijkstra over and there will be some dialogue and a scene.
When you regain control of Geralt you're going to have to head over to Margrave Henckel's house and use Aard to let yourself in. Search the whole house for clues, most importantly the wine bottle and mysterious note on the second floor. Place the wine bottle back on the shelf after you pick it up to open the secret door in the other room.
In here you'll find more clues and a note letting you know who was in on the heist. Before you can finish reading the note you'll hear footsteps downstairs, Dijkstra and Triss are down there and your next quest step involves talking to them. After the conversation you'll have to meet Triss by the Eternal Fire Shrine in the harbor around midnight. Head over there and meditate until midnight, once it's midnight Triss will approach for a scene.
She will come up with a plan to be your hostage to get into the Witch Hunter's Outpost. You'll have a lot of different dialogue options inside of the outpost and one wrong move will bring about violence. Here is how you will want to answer all of the dialogue options to successfully complete this area and learn all you need to:
Dialogue Options to Reach Menge & Learn Everything: "Wanna talk to Menge" -- "Got something to offer Menge" -- "I'll tell your boss, no one else" -- "Fine, take her" -- "Why not" -- "Havent been paid" or "silver sword" -- "Triss betrayed me" - "Dandelion"
If you answer all of these correctly then you'll learn all you need to know about Dandelion and the quest will end shortly after leaving the Witch Hunter Outpost. One of Djikstra's men will stop you and ask you to come speak with him. If you did all of the above dialogue options correctly then all you can give to Dijkstra is the key that you found. If you asked Menge about the treasure instead then Djikstra will be much happier and there will still be some quest left for you to do - involving finding out about Dandelion.
Note: The only way you can get Dijkstra's help in a major future story quest is by asking Menge about his treasure. If you don't ask about it then Dijkstra will refuse to help you when it comes time. For more information about this check out my Brothers In Arms: Novigrad Quest Walkthrough.
For those of you who got into a fight at the Witch Hunter Outpost, you'll have to find a letter in Menge's room which will direct you to a new waypoint on your map. Use your Witcher Senses here to find a drop box on the back of the pillar on the right facing the wall. Once you've discovered the drop box mediate nearby until almost midnight then visit the quest location and hide behind the wall for a scene.