If they have no probable cause for a K9 unit or if you refuse a search and if nothing illegal is in plain sight, it is an illegal search and seizure. Anything they find should not hold up in court.
That's not entirely true.
A dog sniffing your car from the outside requires no probable cause greater than a legitimate traffic stop.
It only becomes unreasonable if it materially lengthens the duration of the stop i.e. you have to stand around cooling your heels waiting for the dog to get there.
Illinois v Caballes If that dog's training is up to snuff, the dog's alert is sufficient probable cause to rummage through your vehicle.
Florida v Harris 
