Below is the list of safe locations and combinations:

Safe 1:

Mission: Dishonored

Combination: 451

Inside the sewers past the area where you get your gear back

Safe 2:

Mission: High Overseer Campbell

Combination: 287

Found inside Dr. Galvani's house

Safe 3:

Mission: High Overseer Campbell

Combination: 217

Found in the locked kennel door in the overseer's base

Safe 4:

Mission: High Overseer Campbell

Combination: 203

Found in the overseer's base

Safe 5:

Mission: House of Pleasure

Combination: 879

Found in the artist's house

Safe 6

Mission: The Royal Physician

Combination: 473

Find Practchett's safe in the North End map

Safe 7:

Mission: The Royal Physician

Combination: 294

Found behind a painting in the destroyed building outside of the Sokolov estate

Safe 8:

Mission: Return to the Tower

Combination: 935

Found in Lord Regent Hiram Burrows' private quarters in Dunwall Tower