Tell tail give aways that i've spotted on unmarked VRS's, A4 Avant, V50's & V70's, Focus ST's & Jag XK's
Usually but not always have stubby roof aerials, additional small black boxes near rear head restraints which light up red / blue!, a second rear view mirror on the windscreen and a smaller wing mirror above nearside wing mirror.
I've noticed a lot of forces are choosing Audi's of varying flavours recently too.