The term for your search is called TSCM or Technical Surveillance Counter Measures. Although there are tools that cost tens of thousands of dollars at companies like REI (http://www.research-electronics.com) or consultants that can do the sweep for you for hundreds of dollars, doing your own sweep is rather easy and free. Depending on the jurisdiction, Law Enforcement may or may not need to get a search warrant to place a tracking device on your vehicle. Knowing this information about your state may help figure out where to start looking for a hidden device. If search warrant laws are lax, then a good visual sweep of the underside of the vehicle and trunk can usually find a GPS device for the following reasons:Law Enforcement may have limited access to your car in the parking lot or while you are shopping, so placement is quick and sloppy and removal needs to be easy. How long the device runs (Does it need batteries which last a few days in real time monitoring mode or wired directly to the car battery). See above. Metal needed for most devices to stick to magnetically. (Although I have used duck tape and zip ties). Bumpers, car frames are great. Near the road surface or direct sky for the GPS radio singles to receive well. (Although cell and wifi gps systems are becoming popular). Edges of the vehicle, bumpers, hoods, truck lids. Vehicle Space. As shown in the recent finding of a FBI GPS on a Santa Clara students vehicle, the devices the SJ or SF FBI field office are using are OLD and BIG. Pic: When you do your search look for anything that might be the size of a pager or . above, often times attached to a brick or tube of batteries. In the maze of moving parts under a car may sound like a needle in a haystack, but trust me, the devices stand out like sore thumbs. LE just assumes no one looks under their car and the device is only used for a few days. If your car was stolen, impounded or there are strict GPS laws for law enforcement in your state a more details search is needed. In those cases GPSs can be hard wired into the car battery and hidden in a million different places that only TSCM tools might find quickly. Remember GPS units can be small (and useful tools) but they cost money that local PD (and SJ or SF FBI) don't have for low priority cases. So your best beat is to just look under the car for that 1995 brick device and get a lawyer.If you are talking about a listening bug, its not there. See the above reasons and then multiple it on complexity of installing it and transmitting the recordings from a moving vehicle. Even with sim card bugs on the market they aren't putting that in your dash to listen to your phone calls they can already monitor.