Have you checked your BB Code Manager to see if it was added that way?
The other place you might want to look, is in /includes/functions_editor.php, at function construct_editor_extra_buttons($editorid)
Or look at the construct_edit_toolbar(...) function around where it says
Code:
// get extra buttons... experimental at the moment
$vBeditTemplate['extrabuttons'] = construct_editor_extra_buttons($editorid);
(Line 372 on vBulletin 3.5.1)
You should also check your plugins to see if any of them hooks 'editor_toolbar_start'