To quote the 4th Amendment ...
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
However, the warrant requirement of the 4th is not absolute. A number of exceptions to that requirement have been recognized by the courts, based upon such factors as whether it is reasonable under the circumstances for officers to obtain a warrant, and whether evidence might be lost or destroyed before a warrant can be obtained. One exception that has already been established by US courts is .... Border searches. The person or property of a person is searched when they attempt to enter the United States from a foreign nation or territory.
It's up to the courts to decide if buses traveling in proximity to the border present a probable cause to conduct a warrantless border search.