Daud in his hideout.
Notice from Daud is a written note in Dishonored.
Transcript
If you have an assignment that requires that you travel to Rudshore Gate, see me. I hold the key.
- Daud
Location
It can be found in the Chamber of Commerce building during The Flooded District. This refers to the key that Corvo will need in order to escape the Flooded District quarantine and return to the Hound Pits Pub.