The Arnolfini Portrait (detail) by Jan van Eyck (1434) - National Gallery, London
Flemish paintings have always been celebrated for the beauty of the details. In the Arnolfini portrait the miror on the wall reflects a whole new scene, probably the painter with another person (his own wife ?).
In Le prêteur et sa femme (1514) - Musée du Louvre, it reflects the outside through the window and an extra person which is not present in the painting itself.
These details in mirors remind me of this photograph of the master of photography August Sander. In The right eye of my daughter Sigrid (1928) - Moma, one can see again as in the Quentin Metsys painting a window opening the scene on the outside.