In harry potter and the chamber of the secrets
we find this conversation when Ron and Harry have taken the polyjuice potion and they go to the Slytherin Common Room
"Yeah. . ." said Malfoy. "Luckily, they didn't find much. Father's got some very valuable Dark Arts stuff. But luckily, we've got our own secret chamber under the drawing-room floor
So ... IF voldy could give Lucius one of his Horcrux(The Diary) then why not the other...and he might have hidden it in that secret chamber...
What say ???