When modifying a host taking it out of a hostgroup, the auditlog report a HostGroup modification, but the data reports only the id of the relationship (the id of the record that used to have the Hostgroup id and host id), so you don´t know the host that not longer belong to that Hostgroup.
If the audit log, instead of just storing the relationship id, stores the hole data of the register ( relation id, hostgroupid and hostid) this will be very handy.