Well I know canine officers use weed balm in ChapStick containers and smear a bit on vehicles they want the dog to alert on. Sure not every canine cop does but some do. The problem is the dog are always rewarded when they do what the handlers desire.
Just to be balanced the cartel use the same trick on border crossers from Mexico. They mark every car with weed balm causing dogs to alert on unsuspecting innocents.