Dirty ventilation ducts could contain trillions of old virii (whether still viable or not is unknown) that are small enough to pass through the filters and are detected in the tests.
I saw an interesting blurb somewhere. Maybe someone else can remember better details. Dont remember where I saw it. Maybe texags site
Basically the gist is..that Cruise ship was the perfect place to tell us about transmission, circulating air, etc.
And the ‘burnout rate’
about 20% got it and that is in line with typical “burn out” of a virus.
My memory on this is sketchy so ....