Hiding Place is a written note found in Dishonored.
Day 15, Month of Timber
I managed to steal away one of the charms they were smashing in Warehouse A. Smashing them! Such beautiful and powerful things and my brothers have no idea. They'll never find me back here though. Nobody ever comes back here. I can brick up the door and they'll never find me. It's all mine.
It can be found in the locked supply depot in the Back Yard of the Office of the High Overseer during the High Overseer Campbell mission. The room is accessed either by climbing around the outside of the building or by finding and using the replacement valve, as per High Artificer Bartholomew's instructions.