Pyres of Novigrad - Novigrad
For this quest you will need to head to Novigrad which is a very large town at the northern most end of Velen. Upon arriving in Novigrad you'll want to travel to the northern portion of the town where you will find Hierarch Square. There will be quite a commotion in the center of the square so head on over for a cutscene.
After the cutscene you'll want to head over to your quest marker where there will be another cutscene. Be nice to the looters and don't provoke them and they'll tell you all you wish to know before Caleb Menge shows up. When the cutscene is over go into Triss' old house and investigate. Below is a list of all the things that glow red with your Witcher Senses:
1st floor: Herbs and Hairbrush
2nd floor: Amulets on counter
3rd floor: Triss' clothes; Rose of Remembrance on the table
Make sure you grab the Rose of Remembrance as it's used in a quest, the rest of stuff is optional for you to find. There are a ton of books and some crafting diagrams for you to collect while you're here too, be sure to grab all you can. When you're ready to proceed you'll want to either follow the quest's guidance (you have a few different ways of getting to our next location) or just cheat and use my screen shots below.
Either use the Signpost or run to the southern portion of Novigrad, specifically Glory Gate. Jump into the water and turn around and you'll see what I am looking at in the screen shot below. This is the secret entrance to Putrid Grove.
Inside of the tunnel you will have to kill a pack of three drowners and loot the key from their bodies. Go through the door and up a ladder to fight some theives, they'll beg for mercy when you almost defeat them. Follow the white dots on your compass to find Triss and Francis Bedlam, the King of Beggars.
There will be more cutscenes after which you will want to follow Triss to another house where you'll have more scenes. After which you will need to escape through the sewers, to open the secret passageway you'll want to pull the brick on the central pillar nearby (use your Witcher Senses to find it).
As for the sack in the water you're looking for, it's on the western end of the quest circle, as shown in the screen shot below. Grab the chest in the water too as it has a crafting diagram in it. Return to Triss and follow her to the next question location.
Inside of the building you'll have to search for piles of rat shit. After finding all three, speak to Triss and watch another cutscene. You'll have to fight some Witch Hunters after it is over, once they're out of the way speak to the guy who hired you and then the quest finally ends here.
Completion of this quest will begin the followup, Novigrad Dreaming.