The folks who tested negative on the home test and then positive on the PCR, did they continue using rapid tests? Because there is a thing with vaccinated folks, that you may show symptoms before testing positive on a rapid test and the thinking is that you may not be shedding enough virus to test positive or to be infectious during those early days. Which is totally different from what they said originally in unvaccinated people about being most contagious right before symptoms. There still seems to be a general consensus that if you test positive on a rapid test that means infectious. Negative does not. Although it could also be true false negatives your friends had.