In my case, I had a login item that wasn't in either my user preferences (
~/Library/Preferences/com.apple.loginitems.plist) or the system preferences (1~/Library/Preferences/com.apple.loginitems.plist`), so none of the prior answers helped.
It turns out, I had something in
/Library/StartupItems/. In my case, it was
MySQLCOM, presumably put there by an old version of the MySQL installer (I've since then installed a more recent version via Homebrew, which does not put anything in that location). I had deleted that version, but the StartupItem remained, pointing to a now non-existent application.
MySQLCOM, restarted, and the Unknown item was gone.
So, check that folder, and see if you have anything unexpected that could be pointing to an application you've since deleted. Also, check
~/Library/StartupItems/, though on my Mac, that folder does not exist.
But follow the other suggestions (looking at
loginwindow.plist), as well, as this Unknown problem can have multiple causes.