I'll take that one.:)...
They would have to first be purchased before being retired. That is, unless they are loose shares found during the course of some sort of audit. Whether on the books and not paid for or through some other book keeping screw up, once found they can either be returned to treasury, for possible resale, or retired, altogether.
I'm no regulations Yoda, so if anyone has a better answer, please do edify we, the unwashed masses.:o)
Lou