I just had an issue with this Mac, it was sleeping and when woke instead of showing the user's login screen who was logged in presently... it displayed 'System Administrator' login screen. There is no user designated 'System Administrator' on this Mac, the single Admin account has a different name and icon. It wouldn't allow you to select switch user, appeared to be crashing possibly so I restarted. Anybody have any idea what's going on here? My first thought was hacking, but I suspect software problem... recently installed Big Sur a few weeks back.