A simply unacceptable number of "drug" sniffing dogs are really just handler pleasing probable cause machines. They can tell when their handler wants them to alert and a dogs desire to please it's master is hardwired way deeper than any drug detection training.
I'll just throw these few post out there for anyone's perusal. I often like this guys legal writing. I'm sure there are better breakdowns on drug sniffing dogs but his blog was where I started to pay attention to what bullshit drug pups were.