A generalised Bayesian model for the probability of getting a false-positive PCR result

It was a disaster before the New Year that I got a positive result from my predeparture SARS-Cov-2 RT-PCR test at a commercial test centre (Site 1) and had to cancel my international flights for the reunion with my wife. I was shocked by the result as I had been self-isolating for more than 10 days before the test with limited outdoor activities (such as shopping for groceries) and I had no COVID-19 symptoms at all. After this moment of confusion and disappointment, I did a lateral-flow-device (LFD) antigen test at home and got a negative result. The second LFD test on the other day also returned a negative result. In the afternoon of the same day, I had my second PCR test at a different site (Site 2), where a professional swabbed my tonsil and nasal cavity so thoroughly that I even smelled a hint of blood. The result came quickly in the next morning and I immediately booked my third PCR test from another site (Site 3) for the same morning. All the three sites are accredited by the UK Health Security Agency (UK HSA). All my results were reported to the NHS for test and tracing.