Police cars are now high-tech rolling offices w/ a data link back to the station. I'm guessing that they are probably constantly streaming data to and from the station like GPS coords, looking up tags, and maybe even searching for the nearest Krispy Kreme location on the internet. All of this data has to be transmitted via RF. If one knew what frequency was reserved for LE data-link in your area, could you not keep pretty good short-range tabs on your nearest police car? FCC.gov should have all licensed frequencies on file so finding your local PD shouldn't be a problem unless they are using an unlicensed band (like the 802.11b standard, but that might not work because of it's limited range).
Armed w/ this information, once might surmise that a broadcasting police car could be detected, or better yet, triangulated with 3 such receivers at known gps coords in an area.
Hell... if they are transmitting their GPS coords and that could interecepted (and probably decrypted), you wouldn't need to triangulate.
Some may call me a dreamer.. What do ya'll think?