Can you say "fishing expedition"?
I was stopped at one of these "random" checks on the CA/AZ border and was held there for better than half an hour with my mother while they did a "drug search" with a dog. Funny thing, not even a cigarette had ever been smoked in the car, but the dog "triggered" on it. Yeah, right...
False positives from drug dogs are documented to be 80% for some dog/trainer teams.
The highest number of false positives are typically the result when the trainer is told that at least one of the samples in the test run will cause the dog to alert.