That's an interesting case. Similar to red light cameras: If the same work can be done by an officer without a warrant (ie, trailing a suspect, or viewing someone disobeying a red light) then why should automation change it?
Personally I think this should require a warrant. They can assign an officer to tail me all they want - but it will cost them significantly more in time and effort than sticking a device on my car. I want that to be difficult to obtain, only when there's already sufficient evidence that a judge will agree my rights to privacy and search and seizure are worth overriding.