It is possible to detect, track and locate police officers (hidden or otherwise) by using their own radios and mobile data terminals (MDT) as locater beacons.
As an example, say the input frequencies to a particular Motorola 800 MHz trunked radio system used by you local corrupt police department are 811.2125, 811.7375, 812.2125, 813.2125, 814.2125 and 815.2125 MHz, with the MDT input frequency at 815.7375 MHz (all data is taken from public FCC records).
If you where to program your scanner to monitor only the repeater input frequencies, you would know when an officer is near by. A typical 800 MHz Motorola HT puts out about 10 Watts ERP with the mobile radios putting out around 60 Watts ERP. Using that information, and standard free-space or urban-space path loss equations you can calculate the distance to the transmitter, if the received power level and antenna gains are known.
Police vehicles equipped with Mobile Data Terminals are always broadcasting on the MDT repeater input frequency. This is because they need to send acknowledgements (ACKs) when they receive data. By monitoring the MDT repeater input frequency, you can be alerted to the presence of a hidden or approaching police vehicle. The closer they are, the stronger the signal will be.
If you ever hear encrypted voice transmissions (a static burst followed by a beep) on the repeater input frequencies - RUN!