Under the Sales > Refunds section, you'll notice that there are red and green truck icons. Those colors denote the status of the customer return.
A green truck means that the customer has returned the item.
A red truck means that the customer has yet to return the item.
If customers receive a refund and do not return the item within 30 days, then you should receive a reimbursement from Amazon.
Check your Reimbursements report for the Order IDs that have the red truck icon. If it has been more than 30 days and customers haven't returned the item, then you can request reimbursement from Amazon.
You can find that report under Seller Central. Reports > Fulfillment > Reimbursements.