The collection of GPS records from your phone is still quite legal and this would be the license plate scanning equivalent.
Even in the absence of a trespass, however, Supreme Court precedence recognizes a search when the government violates a subjective expectation of privacy that society recognizes as reasonable.
So, if the Government monitors your location by enlisting factory or owner-installed vehicle tracking devices or GPS-enabled smartphones, that could well be a search, but it's still a fuzzy and undecided area of the law.
Sotomayor's concurring opinion in U.S. v. Jones is well worth a read if you are interested in this sort of thing.
"[Location data] generates a precise, comprehensive record of a person’s public movements that reflects a wealth of detail about her familial, political, professional, religious, and sexual associations ... trips to the psychiatrist, the plastic surgeon, the abortion clinic, the AIDS treatment center, the strip club, the criminal defense attorney, the by-the-hour motel, the union meeting, the mosque, synagogue or church, the gay bar and on and on. The Government can store such recordsand efficiently mine them for information years into the future. And because GPS monitoring is cheapin comparison to conventional surveillance techniques and,by design, proceeds surreptitiously, it evades the ordinary checks that constrain abusive law enforcement practices: limited police resources and community hostility.
"Awareness that the Government may be watching chills associational and expressive freedoms. And the Government’s unrestrained power to assemble data that reveal private aspects of identity is susceptible to abuse. The net result is that [location tracking] making available at a relatively low cost such a substantial quantum of intimate information about any person whom the Government, in its unfettered discretion, chooses to track may alter the relationship between citizen and government in a way that is inimical to democratic society.