I think some expansion of this statement is in order. As far as settled SCOTUS cases go, the only reason why police need a warrant to attach a GPS to your car is because they must "sieze" the car (for however brief a time) in order to attach the device. As you alude to, it has nothing to do with the surveillance itself. However this is far from a settled issue. At least 4 out of the 9 SCOTUS justices support a new Fourth Amendment theory called the "mosaic theory", introduced by Alito in US v. Jones (2012). Under this theory, courts would evaluate the actions by government in aggregate to determine if the actions constituted a search. Under such a doctrine, 24 hour GPS surveillance on public roadways may well be illegal without a warrant. This theory has been criticized by some academics as too difficult of an analysis for the courts to handle. Yet it does show that the justices are indeed considering the scope of surveillance and some of them are trying to figure out a way to limit the scope using Fourth Amendment analysis. Alito in particular seems worried about it and he has found support for at least some of his ideas from Ginsburg, Breyer, Kagan, and Sotomayor