[His breath catches, excitement buzzing in the back of his head replacing the faint, distant whispers. Sherlock felt hyper-aware of his tongue, pressing against his teeth and the roof his mouth, wanting to elucidate, to answer but he'd tip his hand, he wouldn't want to have the answers just handed over, unless it's by the unintentional slip up by his opponent]

Archival theft is one possibility, there are scores of works that are not available to the public eye, but it's no less valuable.