Unknown tracking number for return?

Weird situation, had a package show up today with one of my products that I fulfill myself (FBM). No RMA slip, just a shipping label.

It's a USPS Ground Advantage Returns label, but it doesn't correspond with any of my return authorizations. At first I thought the customer maybe just printed their own label from USPS and mailed it back without getting an authorization, but I don't think it'd be easy to do this "Returns" label like this without actually initiating a return.

Any tips on how to figure out what order this was from? The only other piece of information I see available is the ship-from address on this, but I'm not sure how to correlate that without manually scanning through every order...