Is that all a quote from Martin Grime? I'm not sure if exact is the right word. The words "the dogs appear to be extremely exact" is a human thought.
'Can the dog mix up traces of human odours with others that are non-human''
I cannot comment on what the dogs think. However, from a forensic point of view and from confirmations of scientific testimonies, the dogs appear to be extremely exact. But, forensic confirmation is required in all cases so as to be included as proof. The CSI dog is trained using only human blood. And using a wide spectrum of donors to ensure that the dog does not individualize them.
EVRD used to be trained using swine (pigs) as their odour is the closest to that of humans. But most of the time, however, the dog was trained using the odour of a human cadaver. Operationally, the dog has ignored large amounts of animal remains/bones when locating human decomposition.
https://www.mccannpjfiles.co.uk/PJ/MARTIN_GRIMES_RIGATORY.htm