Due diligence and collecting histories

Collecting histories are important. They indicate the routes through the antiquities market. And the collecting histories for objects that have been returned to Italy as a result of the Medici Conspiracy are fascinating. So if, say, a major auction house was asked to offer an object that shared the same collecting history (some continue to use the flawed term "provenance") as a returned object, I would presume that the staff of the antiquities department would conduct a rigorous due diligence process. Not only would they contact the Art Loss Register, but potential buyers would expect these purveyors of ancient art to contact (say) the Italian authorities to ensure that there is no overlap with the three major seized photographic archives. Can we be sure that such a rigorous process has taken place? What can buyers expect?