I am not referring to antibody tests. I am referring to tests using antigen (look for the virus proteins themselves) or PCR (look for the RNA) ...
But after the virus is cleared by the immune system, all of those viruses leave little trails of RNA behind. The RNA gets stuck in vesicles one the cells and it can sit there, in the nose or mouth, for weeks or months at very low levels...
On the other hand, with a very sensitive PCR test, people might stay positive for many weeks or even months bc it continues to detect the leftover RNA from the past infection.
But this can be bad - it leads to unnecessary quarantines of people already past their infection
if doing low frequency screening of asymptomatics with PCR, the majority of people found to be positive will be detected only after their infection.
So in the US we are unnecessarily quarantining millions of ppl, assuming they are infected when they’re already recovered