When the cop pulls them over
I've seen the movie a bunch of times and a lot of the stuff that's convoluted to tell the story works.
The traffic stop makes no sense though. Why did he pull him over in the first place? Yeah he found the porn and missed the body but then just let them go. They didn't get pulled over because he thought something was in the trunk, or at least it doesn't seem they were. So what was the reason they were pulled over to start with and why was the cop fine with letting them go?