It can easily be retrieved electronically with a scan tool.
If we sell a vehicle and the door keycode card isn't with the owner's manual package, we retrieve it and the Used Car Dept. pays for it, not the customer!
The keycode is on a decal on the Central Security Module, which is hidden inside the right rear quarter trim panel just to the rear of the right "C" pillar (behind passenger back door). It's a PITA to get to or read.