Of course, it's not the dog that's racist, not really. However, its handlers might be another matter. And they may have no idea that they're influencing the dog.
To understand how that could work, it all goes back to a 20th century horse called Clever Hans that could perform arithmetic. Clever Hans was a huge hit until scientists eventually revealed that the horse was just picking up cues from his handlers and, shockingly, had no real understanding of mathematics. Many decades later, Dr. Lisa Lit at UC Davis bumped into the story and started wondering whether the concept was applicable to bomb and drug dogs.
And, if she's anything like us, spent a good half hour giggling over the concept of drug horses.
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.
"I'm pretty sure these guys are innocent, but whatever, you're the dude with kibble."