In what circumstances will a police officer stop and search a car and find a handgun in the back seat?

In United States v. Arnold, 338 F.3d 237 (7th Cir. 2004), a police officer stopped the suspect for erratic driving and having a burnt out tail light. The officer saw him turn around, manoeuvre between the passenger and driver’s seats into the back seat, either retrieve something from or place it in the back seat and return to the driver’s seat. The officer asked him to step out of the vehicle and performed a pat-down search, which produced nothing. The officer then searched the entire vehicle. He pulled down an armrest in the backseat, which exposed a channel into the trunk of the car. There, he found a loaded handgun.