USPS will be informed of a rural carrier testing positive either by a public health authority or by the carrier themselves. A carrier who tests positive is not required to provide that information to their supervisor but should provide it to the health unit or District OHNA (Occupational Health Nurse Administrator).
https://www.ruralinfo.net/wp-content/uploads/2021/02/COVID-19-Decision-Tree-January-21_2021-v2.pdf
https://www.ruralinfo.net/wp-content/uploads/2021/02/Decision-Tree-1-21-21-Explanation-and-Q-and-A.pdf