You don’t actually know that for sure though. The following paper found PCR positives months after being infected, with a slowly decreasing odds ratio, which is in their words consistent with shedding: https://jamanetwork.com/journals/jamainternalmedicine/fullar...

Unless someone gets a PCR positive, has the virus sequenced, and then gets a separate positive later, has the virus sequenced again and can confirm they were actually infected again with live virus (perhaps using live viral culture) they can’t really be sure.