What is the test for responding to an anonymous 911 call for marijuana possession?

Most recently in Navarette v. California (2014) ___ U.S. ___ [134 S.Ct. 1683, 1688, 188 L.Ed.680] (Navarette), there was an anonymous 911 tip that a driver of a specific pickup truck with a specific license plate number had run the anonymous caller off the highway at a specific location. Police responded within 18 minutes of the anonymous tip, spotted the truck near the location described by the caller and followed it for about five minutes before pulling it over. As the responding officers approached the truck, they smelled marijuana. The truck was searched and thirty pounds of marijuana were seized from the truck bed.