The safe codes found in The Last of Us Part 1 are easy to miss, especially as they're not always close to the safe they open. It's made even harder because you've got to keep a constant lookout for infected. The rewards are mostly worth it though, and you'll get parts, ammo, and other useful items for your trouble.
There are four safes to find and open, and while the code combinations for each can be found somewhere nearby, you won't actually need to find the items to unlock the corresponding safe as you did in the original.