Fourth Amendment - car search
Even if stated purpose of the search was to look for a gun, the odor of drugs, an observed transaction, and the discovery of drugs in the possession of an occupant gave probable cause for the search.
http://media.ca8.uscourts.gov/opndir/16/01/151135P.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.