Maybee it's a bit late but I just discovered the thread right now:
http://www.theroyalforums.com/forums/394405-post98.html
This picture was taken in St. Moritz at a private party. The party was organized by Christian von Fürstenberg and the theme was "Playboy Mansion". Girls were supposed to dress up as bunnys and the guys as Hugh Hefner. Thats why Pierre and Andrea 8as most of the other male guests) were wearing boxer shorts and housecoats.
The party was held in the privée of Chesa Veglia. This Location is run by the Chesa veglia Club which was founded amongst other by Caroline Grimaldi, Heinrich von Fürstenberg and many others.
As both the party and the location are strictly private there were no journalists and no cameras allowed. So the pictures were for sure made from one of the invited guests.
The party was attended not only by Pierre, Andrea and the Hannover sons but also by Caroline and Ernst August and for sure Tatjana Santo Domingo. Other guests included Rold Sachs (a close friend to the grimaldis), Sven Ley (Escada heir) and other other international celebrity.