Probably not detecting the viral particles directly. Rather some chemical change occuring as a result of infection. Interesting to see if the dogs can differentiate between covid and influenza, etc. I believe they can be trained to detect some types of cancer and not others.
I wondered the same, but they say:
““THE DOGS CAN DETECT THE COVID ODOR ON A COUNTER OR TABLE IF IT WAS RECENTLY TOUCHED BY A COVID-POSITIVE INDIVIDUAL. . . .”
So, maybe my sweat or body oil odor changes?