Putting on my cynical hat, I wouldn't be surprised if a fair number of police K9 handlers (the ones who understand animal training and are capable of this) inte

Putting on my cynical hat, I wouldn't be surprised if a fair number of police K9 handlers (the ones who understand animal training and are capable of this) intentionally train their dogs to alert to some surreptitious signal (in addition to drugs). That way they can get "probable cause" to search whenever they want to, without having the dog alert 100% of the time.