Just googled the gravesite of Maria Callas. Indeed, she DOES appear to be buried in Pere Lachaise (I think that’s the correct spelling).
Maria Callas – The opera singer's ashes were originally buried in the cemetery. After being stolen and later recovered, they were scattered into the Aegean Sea, off the coast of Greece. The empty urn remains in Père Lachaise.
Findagrave,com says:
She was cremated and her remains kept in a niche at the cemetery of Pere Lachaise until the spring of 1979 when the urn was taken by plane and scattered over the Aegean Sea off the coast of Greece by the minister of culture for that country. [The ashes were flown to Greece; the scattering was accomplished from a boat.] A niche plaque was placed as a memorial by the cemetery while indicating the now empty space was the initial resting place for the urn containing the ashes of Maria Callas.
At http://www.death2ur.com/maria_callas_gravesite.htm:
"One of the most renowned opera singers of the twentieth century. The photo on the right denotes the niche where Maria Callas' cremated remains once were interred at the Pere Lachaise Cemetery in Paris. Her ashes were removed and scattered over the Aegean Sea by a Greek dignitary. The plaque remains as a memorial to the legendary singer."
And, by the way, there is a crematorium at Pere Lachaise, which is presumably where Callas' body was cremated - it may have been that for which the German visitors were looking:
http://commons.wikimedia.org/wiki/File:Cr%C3%A9matorium_du_P%C3%A8re-Lachaise.jpg
I don't mean to pick on you - but your post on this subject already appears in a google search and I don't want any more inaccurate information about Callas circulating - there is plenty already. Regards!