How K-9 searches go wrong:
The problem isn't that the dogs aren't capable of picking up the scent, it's that dogs have been bred to please and interact with humans. A dog can easily be manipulated to alert whenever needed. But even with conscientious cops, a dog without the proper training may pick up on its handler's body language and alert whenever it detects its handler is suspicious.
In one study published last year in the journal Animal Cognition, researchers rigged some tests designed to fool dogs into falsely alerting and others designed to trick handlers into thinking a package contained narcotics (it didn't). Of the 144 total searches performed, the dogs falsely alerted 123 times. More interesting, the dogs were twice as likely to falsely alert to packages designed to trick their handlers than those designed to trick the dogs.