In Almeida-Sanchez v. United States, supra, the question was whether a roving patrol unit constitutionally could search a vehicle for illegal aliens simply because it was in the general vicinity of the border. We recognized that important law enforcement interests were at stake, but held that searches by roving patrols impinged so significantly on Fourth Amendment privacy interests that a search could be conducted without consent only if there was probable cause to believe that a car contained illegal aliens, at least in the absence of a judicial warrant authorizing random searches by roving patrols in a given area. Compare 413 U.S. at 413 U. S. 273, with id. at 413 U. S. 283-285 (POWELL, J., concurring), and id. at 413 U. S. 288 (WHITE, J., dissenting). We held in United States v. Ortiz, supra, that the same limitations applied to vehicle searches conducted at a permanent checkpoint.
It allowed the actions in this case specifically because they were not searches, in the same sense as a traffic stop is not a search, and the police, having pulled your car to the side of a city street for having expired tags, cannot without probable cause demand that you open your trunk to allow inspection.
Martinez-Fuerte.
> Also, I can't tell if you're nitpicking my summary or if you actually do believe there's a 100-mile-wide Constitution-free zone hugging the US border.
“100-mile Constitution-free zone” is a simplification that, like any simplification, is somewhat inaccurate; there is a broad (unclear as to it's maximum limits, the 100 miles is current executive policy) in which the courts have allowed, in fairly broad circumstances, substantially lower standards for search and seizure than are otherwise Constitutionally applicable in the US, rendering the protections of the Fourth Amendment in practice much weaker.