Right, it was when they were trying to remove the tapestry from the wall. It wasn't clear to me if Hermione was part of the conversation, though. I think I'm in the minority, too, in not thinking the locket at Grimmauld Place is the horcrux. It sounded like the horcrux locket had a pretty clear serpent on it, obviously marked as Slytherin's. Hard to believe that if that were the locket in Grimmauld Place, the only description would be that none of them could open it.
I'm open to anything. I think JKR likes to throw us off track sometimes by putting little things in there like the locket at Grimmauld Place.
I don't think anybody would think twice if something at 12 Grimmauld place had a serpent on it, because Sirius' entire family were Slytherin, except for him.