Customer returned an item that isn't from us and received a full refund

We recently had an order refunded as the customer claimed it was defective so Amazon gave them a full refund. The item was returned to FBA and we requested removal.

We have now received the returned item and it is an item that we have not had stock of since March 2022 yet the order was placed for a different, but similar, item in October 2022.

It is impossible that the item has come from us given we have not had stock since March and there are no older orders for this customer on our account.

We have received this with Amazon but we are going in circles … the returned item is currently being sold on Amazon from a company in Lithuania at £19.99 … the item we supplied is UK stock and is £23.95 so effectively they have returned a used £19.99 item and received a refund of £23.95

Any advise as I am 100% sure the returned item has not come from us?