Well, the title 'de Réthy', 'van Retie' was a title members of the royal Family used incognito. The most known case comes to mind when Prince Leopold (future King Leopold III) visited Sweden before his engagement to Princess Astrid of Sweden, he was introduced to friends of Prince Carl & Princess Ingeborg of Sweden (his future parents-in-law) as 'Monsieur Philippe de Réthy', while his mother Queen Elisabeth was 'Madame de Réthy'.
Eric VdV