Safes are locked caches of supplies that open once you find the note or item with their combination. Opening all of them in The Last of Us Part I earns the Sticky Fingers trophy.
The Outskirts: in a bookstore in the subway; the Note to Derek gives 3-43-78. Bill's Town: next to a truck; the Note to Bob gives 5-17-21. Pittsburgh: behind the counter as you enter the Hotel Grand lobby; the Note to Staff gives 22-10-56.