Quest "Whistle for a Thief", after Kerold said that you have to go to the sewers to retrieve it. It's not obvious where exactly whistle is gonna be. I already explored sewer, so I thought it would end up in water (there's money in the water, so it's not unreasonable). After I searched every inch of sewage water, I had to look it up online.

I propose following: make it so "entry point" of stuff was obvious when first exploring sewers. Stream of water flowing from the ceiling, then as brook flow down the corridor and into that big room with pool of sewage water. With water sounds.