Customs officials do not need a search warrant. They can initiate a search on pure whim at any time.
Searches can be initiated because the Customs officials dont like your looks or think you look like a Witch.
Customs officials can initiate searches because you have recently crossed the border several times.
Customs officials can initiate a search because your name or a name similar to yours appears in their secret computer lists.
Customs officials, border patrols, and the police can initiate a search if you are a male in a rented vehicle near the border at night.
Customs officials can search your vehicle with a dog, search your vehicles contents (including locked luggage) with a dog, and physically search your person.
The U.S. government places remote wireless sensors near the border to detect persons attempting to cross the border at unmanned locations. Border patrol officers are stationed along the border and respond to these sensors very quickly.
Border Patrol and other police can search anyone who has crossed the border and not yet stopped and let someone in or out of their vehicle.