There ARE NO related files; that's why your login items are labeled "Unknown". The files to which they were originally related were part of the contents of the apps/utilities that placed them, which have since been trashed.
When you installed those apps/utilities they placed login items in /Users/xxx/Library/Preferences/loginwindow.plist, and when you uninstalled them the login items remained, but with no sources, hence the "Unknowns".
This is probably one of the reasons Apple has deprecated Login Items and asking developers to move to "Launch Agents".
