So...Sorin came to Innistrad to check up on Avacyn, right? What's he been doing there this whole time, anyway? Because it seems like the only one actually doing anything plot-related is Liliana, who actually went out and broke the Helvault. If Sorin were really there for Avacyn, he would have been more involved, right?
So here is my theory. Look at the three abilities on Sorin, Lord of Innistrad.
-2: Train your vampires
-6: Turn some people into vampires
He's all about recruiting and training. You know what this means? It means Sorin actually came to Innistrad to build an army!
Of course, this raises the question of what he's going to do with an army. Take over the plane is my guess. He could be Sorin, Overlord of Innistrad. In any case, though, I can't see how making vampire tokens, pumping them, or gaining control of creatures is going to do anything related to finding Avacyn, so he's got to be hiding something.