Look up 'NYPD Taxi Police'. They look just like NYC taxis but they are fitted with lights and siren and they have on duty officers in them.
My local PD is pretty sly too. They have an unmarked gold Astro Van, an unmarked silver Taurus that is impossible to identify as a police car until it lights up in your rear view mirror, and a Chevy Uplander minivan complete with stick family stickers on the back!
Yep, seen this kind of thing too. In town and on freeway. Deadly because you'd never know until the lights come on...