The thief left DNA evidence at the scene and eventually pleaded guilty. The person in TFA didn't "find" the artwork, someone came to him with it in order to arrange a handover without getting into legal trouble themselves.
How would that work? Police: Hey thanks for returning stolen property, since it was in your possession we will charge you now, that is unless we learn who gave it to you.
The hard part, as with most transactions of this sort, is trust. I'm curious how Brand convinced the returner he wouldn't double-cross them.