Ever heard of onstar for your car,all they need is a police officer to OK it and they can listen,track,stop your car all without consent as long as they can prove you are doing something bad is all it takes.And yes i do know about the guy who got his stolen car recovered by this means but if that is what it takes then you can have my car that is what insurance is for.
Better yet, how about a device which can discern criminal thoughts germinating in your mind and stop you from commiting the crime before you do it by pre-emptively sending a police officer to pull you over?