Ths most common cause of files with unknown UIDs is because the previous owner account has been deleted.
Files outside the home directory and a few other locations such as the mail spool are not automatically removed when a user account is removed.
Who installed moodle, or who has had access to the html tree?
It is also worth noting that tar will allow you to open an archive and keep the original uid/gid - which may not exist on your system. It is possible that an ill-behaved installer may have created those with a non-existent owner.
Last edited by astrogeek; 01-12-2015 at 03:02 AM.