Just because the first part of the reg number is the same doesn't necessarily mean that its the unmarked car your dad has seen in the petrol station.
Bear in mind 'unmarked' police cars aren’t exactly secret squirrel stuff; they have lights in the front bumper/grill, a camera under the rear view mirror and a host of lights, cameras, antennas on the rear parcel shelf. In short, they stick out like the proverbial dog's doodads, even at night.
Typical barry boy BMW 5 series probably.
Detritus.