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.
Good point. I think it's interesting that the locket could not be opened, especially in light of how HP knew that the locket DD retrieved from the cave was a fake because it was open.
I'm still waiting for books 1-4 to be returned to me. I need to read them again, slooowly and carefully!