Item description:

No Stone Unturned Collect every single remnant.

Save loads at the start of the Sewers portion of the game. I can't really provide a surefire guide as to where to find the last remnant as the game procedurally generates the areas and remnant spawns, so where I found my last remnant likely won't be where you'll find yours. Luckily, for me, it was the very first one I came across and was a red sealed remnant. My best advice is to thoroughly search and grab each remnant you see. Trophy will pop once it's the correct one.