For a Drupal site I was developing I originally tried out the Acquia distribution but ultimately dropped it in favor of the vanilla core release. In doing so I also removed the custom Acquia modules that provided integration with their network as I wasn't going to be using it either. All was good, except that the admin menus retained an unneeded menu item named "acquia_subscription_status" that just linked to acquia.com - I had to vanquish it!
Having removed all of the custom Acquia modules I was confused as to why there was this random menu item hanging around. It wasn't part of any of the contrib modules I was using, so I grep'ed my entire site to see if something remained - it was clean. Given the admin_menu module works off the normal menus I then tried the configured menus, again to no avail. So that just left the database.
In the database it seemed fairly obvious that it should be in the menu tables, and sure enough it was. The menu_links table included a menu item that went something like this:
Sure enough, this was it. So then I just removed the menu item and..
It was still there. :-|
A quick look identified the simple fact that the menu was cached! Duh.
So, a quick "TRUNCATE cache_menu;" (or Flush All Caches -> Menu) later and life was good, the menu item was gone.