Credit to theLoadout.com
Here are the three Modern Warfare 2 safe codes and the answer to solving them in each level:
02-02-19 – El Sin Nombre safe code (The safe is in Diego’s room. It can be opened by looking at the date on the portrait when in the same room.)
10-10-80 – Alone safe code 1 (The safe is in the room past the dying man when you open the door. You need a blade to pry open the door. Can be solved by looking at the calendar in the room.)
37-60-80 – Alone safe code 2 (The safe is later in the mission, in the garage. Look at the computer in the mechanic shop to find the code.)