hmmm, thats an interesting thought. I am sure there are police officers out there that own all sorts of cars and drive them to and from work in their uniforms. I wonder if they have the ability to book you when they are off duty?
I knew a guy who owned a VL calais, the series 2 version with the transparent grill. He put 4 strobe lights bhind it - a blue one, two white ones and a red one... He used to pull people over at night time for laughs (apparently a big offense). But if anyone saw this they would say "I saw a VL cop car pull someone over, they drive anything these days!".
I have not seen a legitimate undercover cop car without the standard black on yellow plates, thats my theory.