It's the nuances of their relationship.
Neal: Is Franklin talking to his back up? Is this a CI off!?
Peter: Hard to say! He's reading her the VIN number!
Neal: Wait, you said Volder kept evidence against Decker.
Peter: Yes, in his office.
Neal: So he's the kind of guy who likes to have a backup plan.
Peter: No! I already checked! No key in the obvious places!
Neal: Wait--wait a second. There's a small recess inside the rear bumper molding--it's where I'd put a backup.
Jack: Peter, she found the code, but it's going to take a couple of minutes to see if it works.
Peter: *Starts the car, grins* My CI beat your CI.
Jack: Yeah, well, mine's out of practice. *Jumps into Peter's car*