One of the most interesting aspects of this question, based on my experience, is whether the Product Owner of the team "should" attend the Sprint Retrospective. On many occasions, I've heard it said, either by somebody on the team, or by a manager, that the PO should not attend the retrospective, and I must confess, I've always found that perspective to be potentially concerning. One of the anti-patterns I've observed, albeit rarely, that helps add clarity behind this perspective, is a situation where the PO has a direct report who is a member of the team, which tends to reduce the potential for true psychological safety. Leaving that rare situation aside, it's my preference that the Product Owner be present, and when asked, that's always been my position; I'm curious about the perspective that others have on PO attendance, in particular.

Philip Rogers
Agile Coach