There are several ways for an LEO to legally search a vehicle..
1. WIth consent
2. With a warrant
3. Without a warrant, but with probable cause prior to the search starting
4. An inventory search of a vehicle being lawfully impounded.
Now, I can't think of a scenario where a cop would get suspected drugs to field test from a car without having already initiated a search. Now, bend the OP story a bit. Cop standing at the door of the car, sees this flake on the floor. If the nature of the flake as cntrbnd is immediately obvious, the officer could seize the flake. He could then do the field test, get a positive result, and use that as probable cause to conduct a search. IN my opinion, this scenario won't have an issue at the field test, but rather when the officer tries to explain how a little hunk of crap on the floormat of a car was immediately obvious as contraband allowing its seizure. I'd love listening to that testimony.