It's helpful to continue to think about the current possessor's position.
I think there should be two foci. One is the one you describe -- on the current possessor. It would be wrong for that current possessor to hold the watch silently or to try to sell it. Hopefully Antiquorum has provided (or will provide) the current possessor/consignor with details of the theft and with Nico's contact information.
The second is to continue to focus on Antiquorum, which has done several things wrong that they can still correct. Antiquorum can provide Nico with the identity of the consignor. They haven't done that. Or, if Antiquorum doesn't want to help Nico get his watch back, then Antiquorum can make Nico whole monetarily based upon the decision that Antiquorum made: Their New York office received the stolen watch back from the honest buyer, gathered the information from Nico that the watch was stolen from him less than 5 years ago, and then chose to immediately send the watch back to the consignor rather than hold it (pending legal resolution) or involve law enforcement. As was stated in another thread, Antiquorum prematurely acted as both judge and jury. Antiquorum can and should step up to the plate here and compensate Nico. We're talking about approximately $40,000 USD (the approximate replacement cost for a Langematik Anniversary with box and papers), not a million dollars. If Antiquorum were to do this soon, they would look very, very good.