In Dishonored, all of the safes in the game have combinations that you can find, typically in the vicinity of the safe.
Sometimes the clues will be in different places on different playthroughs as well, which I am definitely a fan of.
Most combinations are on a note in the area, or a clue will be on a note pointing you to another note or book. Some can be more subtle, like an important date in the characters life that translates into a three digit number.
The basic premise here is, look around. There are always clues if you look around. They can sometimes be hard to spot, but they are there somewhere.
If you just want some codes without doing any work at all (you lazy bastards!), then check out my other article for all of the safe combinations in the game.