The PCR-test uses primers to identify genetic material that belongs to SARS-COV-2.
It turns out that one of those primers was not a viral sequence, but a sequence from genetic material every human being has: chromosome 8.
In other words, that primer will always be positive, infected or not.
WHO PCR-test primer sequences for COVID-19 detection PDF
In the table in the 2nd row, look for this information:
nCoV_IP2-12759Rv – CTCCCTTTGTTGTGTTGT
This is chromosome 8 which you can verify in GenBank.