I did some research and found this 1973 SCOTUS case that matches your description:

http://scholar.google.com/scholar_case?case=6933260753627774...

Here is the key quote, taken from section 273:

  But the search of the petitioner's automobile by a roving patrol, on a California road that lies at all points at least 20 miles north of the Mexican border,[5] was of a wholly different sort. In the absence of probable cause or consent, that search violated the petitioner's Fourth Amendment right to be free of "unreasonable searches and seizures."

Edit: Reading through the decision, sections (pages?) 280-285 briefly summarize some of the conditions under which warrantless searches are permissible. This is very interesting reading, thanks for the heads up!