Well I read on BBC website a report that a care worker at the Isle of Skye - where they had an early outbreak and subsequent critical report - has just tested positive again. She had it in the original outbreak, test confirmed, then had two negative tests, now positive again.
Think medics don't know if you can catch it again or it decreases in your body then flares up again (symptoms plus antibodies).
In China they noted people testing positive a few months later but found it was dead cells in lungs not a new infection. As the test is only 75% sensitive the negative ones may be false negatives. Difficult to say.
I know people who have had positive antibodies a few weeks after having covid, then testing negative 6weeks later