After reading a couple peoples' ideas, especially Paul Levinson's, here: link
I think it's obvious that Sean is it. A lot of people are saying he's acting TOO guilty and too suspicious, so it'd be a cliche that, of course he'd be it.

But they've set it up really carefully. It's key enough to where people are assuming its just the cliche that they dismiss him. But that's the trick. When everyone is focused on people who it must be, since it CAN'T be Sean because they've made it TOO obvious, they sneak around and actually make it sean. Thoughts?