Well it is not far fetched that they can smell the excitement or anxiety of their owners as they approach the evidence they believe is the correct one?
I don't know how good dogs are at pose detection, but it's definitely plausible that they can tell when an alert is expected.
It's less about the handler's pheromones and more about movement, speech and posture.
Dogs can however distinguish behavior patterns in their handlers, because that's mainly how they get rewards and are punished.
Both the patterns and the rewards are much more subtle than handlers are generally aware of. A service dog is in some sense the worst case of an overfitting machine learning model you can imagine.