FOUND IT! Thank you! It was on a wooden coffin (the only one in the game) hanged in the sewers! Precisely on the left when you exit from the lower passage of the zone where you find Eileen. The zone where is the Insight point :D. Thank you sincerely :D
I tried to be as accurate as possible to help others find that hat :D . I spent 3 days searching that hat in my other runs. I found it in my first playtrough with luck, and i was upset for not finding it anymore hahahaha.
