Your EMS specifies controls essential for achieving your environmental objectives (and targets) and for addressing the processes, products and by-products responsible for your organization’s significant environmental impacts (both beneficial and adverse).
These controls may be associated with the selection and management of your suppliers. This may be where your colleagues decide that audit is the preferred way of managing the risk of pollution or increased global heating that could be caused by suppliers fulfilling your requirements.
Only in these circumstances would supplier audits be required by ISO 14001.