Considering the dog’s far superior sense of smell, it would almost seem unreasonable to challenge their participation in the investigation of crimes. However, these animals—like people—are not perfect. Even if a K-9 is in fact correct about detecting the scent of a controlled substance within a vehicle, the odor of such substance can be present hours or even days after the substance has been removed from the vehicle. Also, since drug dog handlers are required to maintain records of the success rates of their dogs, too many indications that don’t result in the discovery of drugs can deem the dog unreliable. This circumstance can help a defense attorney suppress any evidence obtained by the dog due to unreliability of the dog’s past endeavors.
Furthermore, even the best drug sniffing dogs can make a mistake. False indications happen. Thus, indications of false positive, whether by virtue of a mistake or the odor being present but not the substance, can be grounds to dismiss the reliability of a drug sniffing dog. Similarly, because each drug sniffing dog is usually assigned a dedicated handler, biased behavior by the handler can be misinterpreted by the dog and result in a false indication. There can also be deviations in training methods among the various agencies, as well as variations in the training performance of a drug-sniffing dog.