Quoting Obi Wan (loosly): "I was under the impression that Sifo-Dyas had been killed (10 years ago, as indicated by Obi Wan earlier to the Kaminoans)..."
Yoda & Mace: "......."
Why silence? Why did they say nothing ion reply? Why couldn't they confirm Obi Wans "impression" as being either true or false? Could they know something that they're keeping from Obi Wan?