Corvo's Bounty Note is a written note in Dishonored.
Finding Corvo in possession of that mask could have been an elaborate frame job, but now the newly minted Lord Regent Havelock is announcing to all the city that he has proof linking Corvo to the Masked Felon.
The bounty is 30,000 coins, but if Corvo truly is the Masked Felon and is responsible for all the things the wanted posters accuse him of, I expect the bounty to double. Because we have him alive, we should be able to barter even more.
Regarding Slackjaw, we watched him for a week, and when we were ready to make our move, he suddenly disappeared. Maybe he left the city.
It can be found on a desk in the building where Corvo is imprisoned during the mission The Flooded District.