I agree.Pretty much any patrol car I ever drove would get a hit by a dog. Cops frequently transport drugs in their cars...it's part of the job. So that would hardly be a valid control.
Which illustrates one of the problems with the dog's reaction as establishing probable cause. There are many reasons that might cause a dog to react in a particular way that do not necessarily indicate probable cause to believe a crime has occurred.
Further, how does the defendant cross examine the dog under oath?