Does LE need a reason to do a free air sniff of a vehicle?

It depends on where the vehicle is located. A free air sniff of a vehicle in a public place is not a search, so the Fourth Amendment is not implicated. See United States v. Caballes (2005) 543 U.S. 405, where the U.S. Supreme Court held that the use of a well-trained narcotics detection K9, one that did not expose non-contraband items that otherwise would have remained hidden from public view, during a lawful traffic stop, generally did not implicate legitimate privacy interests. However, if the reason the vehicle is in a public place is because LE conducted a traffic stop, the sniff must take place during the time necessary to complete the mission of the traffic stop, unless additional probable cause is developed. If a vehicle is on private property, particularly within the curtilage of a home, a sniff would implicate the Fourth Amendment.