As discussed in the comments of Does normal paper currency contain enough narcotics residue to attract a drug-sniffing dog?, and further explored in the 2017 NPR article, "Eliminating Police Bias When Handling Drug-Sniffing Dogs" on Dr. Lisa Lit's UC Davis paper, "Explosive- and drug-sniffing dogs' performance is affected by their handlers' beliefs", there is some evidence that drug-sniffing jobs may be unduly influenced by their handlers.
On the surface, the study tested the abilities of fourteen certified sniffer dogs to find hidden "targets." In reality, the dogs' human handlers were also under the magnifying glass. They were led to believe there were hidden target scents present, when in fact there were none. Nevertheless, the dogs "alerted" to the scents multiple times — especially in locations where researchers had indicated a scent was likely.
This was also noted in a Cracked.com article on crime-fighting tactics that don't work which also quoted Dr. Lisa Lit's study.
So Lit set up a room complex where the dogs would be presented with multiple scents of interest (read: sausages everywhere), but no actual drugs or explosives. However, the handlers were told that they were looking for the real thing, and also that the areas with conflicting scents were marked in a certain way. The results were condemning: Only 21 out of 144 searches accurately reported nothing of interest. There were a total of 225 alerts from the dogs, each one of them a false alarm. In areas with the fake marking that the trainers were told about (and were therefore extra wary of), the dogs were twice as likely to give a false positive.
Given even the best-designed study can come up with bad results, have Dr. Lit's studies results been reproduced by other researchers?