Michigan State Police also said the man showed signs of impairment after approaching the suspect vehicle.
The traffic stop happened in Oak Park on Aug. 21, according to tweets posted by state police in Metro Detroit.
According to troopers, the 28-year-old driver was pulled over for improper driving and operating a vehicle without insurance.
While next to the suspect car, police also spotted a 9 mm Taurus firearm with an extended 33-round magazine in the vehicle's back seat pocket. It was within reach of the driver, police said.
The driver also liked about having a concealed pistol license before being asked to perform a sobriety test.
He was later arrested.