(II.G) If a member museum, as a result of its continuing research, gains information that establishes another party’s right to ownership of a work, the museum should bring this information to the attention of the party, and if the case warrants, initiate the return of the work to that party, as has been done in the past. In the event that a third party brings to the attention of a member museum information supporting the party’s claim to a work, the museum should respond promptly and responsibly and take whatever steps are necessary to address this claim, including, if warranted, returning the work, as has been done in the past.
Thus information about the recent surfacing of an object could be derived from the museum's own investigations or from a third party researcher. Note that the obligation is on the museum to respond: not for the "source country" to make a claim.
Minoan larnax photographed while passing through the market; Minoan larnax in North American public collection.