There are numerous safes around Seattle and one in Jackson. Some require finding an artifact (piece of paper) that denotes there is a safe nearby before finding them. Here are all of the safe combo numbers once you find them. If you happen to miss one, you can simply use Chapter Select and replay the level to find and open them.
I would recommend using this playlist as a guide for ones that you have missed/left due to having to fight infected to get to a safe (credit to PowerPyx):
Safe Codes:Chapter 4: Patrol (Jackson) – Safe Code: 07-20-13
Chapter 8: The Gate (Seattle Day 1) – Code to Electric Quarantine Zone Gate: 0512
Chapter 9: Downtown (Seattle Day 1) – Safe Code (Bank Vault): 60-23-06
Chapter 9: Downtown (Seattle Day 1) – Safe Code (Madison Street): 04-51
Chapter 9: Downtown (Seattle Day 1) – Safe Code (Courthouse Office): 86-07-22
Chapter 9: Downtown (Seattle Day 1) – Code to “Fuck Fedra Gate”: 5345
Chapter 11: Capitol Hill (Seattle Day 1) – Safe Code: 55-01-33
Chapter 13: The Tunnels (Seattle Day 1) – Locker Room Code: 15243
Chapter 16: Hillcrest (Seattle Day 2) – Safe Code: 30-82-65
Chapter 18: The Seraphites (Seattle Day 2) – Safe Code (Apartment Building): 08-10-83
Chapter 18: The Seraphites (Seattle Day 2) – Safe Code (Weston’s Pharmacy): 38-55-23
Chapter 21: The Flooded City (Seattle Day 3) – Safe Code: 70-12-64
Chapter 25: On Foot (Seattle Day 1) – Safe Code (MS Corporation Soft Drink Distribution Center): 17-38-07
Chapter 28: Hostile Territory (Seattle Day 1) – Safe Code (Jasmine Bakery): 68-96-89
Chapter 31: The Coast (Seattle Day 1) – Safe Code (Ship): 90-77-01
Chapter 33: The Shortcut (Seattle Day 2) – Safe Code: 30-23-04
Chapter 34: The Descent (Seattle Day 2) – Safe Code (Gym): 12-18-79