Secret Stash Tip is a written note found in Dishonored.
I was told you need to turn on the faucet in an old sink three times to open up some kind of secret door.
No information on the value of what's in this room, I only heard about a street speaker hanging over the road, just outside the door. Sorry about that, mate.
Don't forget my share if you find it.
It can be found next to a corpse on a rooftop during the mission, The Royal Physician. It is the first rooftop to cross the street when entering North End.