Is there an update on this?
Seems like Return ID is only available if return is created using API call - https://shopify.dev/docs/apps/fulfillment/returns-apps/returns#step-2-request-a-return.
There seems no way to fetch the return ID if the return was initiated using Shopify Admin UI. And retrieving a return needs the ID -https://shopify.dev/docs/apps/fulfillment/returns-apps/returns#step-2-request-a-return
Same applies to the Approve/Decline operations for Returns, where Return ID is mandatory.
It will be great if someone from Shopify can through some light on this.