I've known people who have had their car searched because a dog allegedly signaled that there were drugs in the car when there were not. They looked like stoners (long hair and tie-dyed shirts) so the cops probably thought the odds were good they would find something. When they didn't they just blamed the dog and said something along the lines of, "well, you were probably smoking pot in this vehicle at some point, and that's probably what the dog smelled."
The dog is just an excuse to violate your rights.
THIS. There are no statistics on how frequently dogs "alert" and the subsequent search finds no contraband. The police document when they do find something in such a search but don't document it when they don't, so the statistics make it appear that dogs are extremely effective. When confronted with double-blind tests they don't perform nearly as well. They also generate a lot of false positives, when training the dog almost always encounters what it is looking for so when they get in the field they tend to generate a lot of false positives. The police are fine with this as it gives them probably cause to perform a search. I'm glad the SC is finally pushing back on this issue.