I have been using a very simple trick with my scanner, that combined with my STi-R and LI, have saved my ass from numerous tickets while travelling on Ontario Highways.
All OPP cruisers are equipt with a "vehicle repeater" which receives the signal from their handheld, and amplifies it out to the tower - and vice versa...
I'm sure you can see where I'm going with this. The "VR" can only be heard about 2 - 3 miles away, so I set my scanner to the known OPP VR Frequency (as listed on radioreference) and voila! Even the most clever officer running K band I/O can't hide from his own repeater screaming out his location.
Yes, there's at least half a dozen reasons you can't always rely on this. First, someone needs to be talking directly to the officer or on a common channel the officer receives so the VR is transmitting. Another thing is this doesn't work well in urban centers as you'll have no idea if the officer is anywhere on YOUR street... but this does work well whenever you're on a road trip in Ontario and is an excellent use of your scanner!
I'm sure other police departments have this lovely feature too... might be worth looking in to!