This is one of those developing discussions in which none of us have much of a clue, it seems!
Anyway, the benefit of having loads of different accounts to compare is that I found another with OSC not installed in the root and that too is without the file /.fantasticodata/installed_in_root.php so, jc8654, it just deepens the mystery that you have it too! You can tell which scripts have been installed because there'll be a folder for each in /.fantasticodata, but these will be empty (if uninstalled).
BTW, you asked what my example installed_in_root.php contains — it's just 6 bytes in size and I'll have to show you in a code window:
