There are some individuals that continue to shed vital particles, maybe even dead particles, for extended periods but at low loads deemed noninfectious. After a positive test, it is recommended to not retest for 30 days in pool testing due to the possibility of continuing to test positive. On average, most will test negative between day 10-14, but there are always outliers.
Alternatively, if this individual never had a positive result, Thy could have had immunity that prevented high viral replication and just kept the virus at bay.
Hope that helps.