[He's about to comment on how the people around don't look like Reapers, but he stops himself before opening his mouth. "Look"? Why was he about to use that word? Why not "they don't feel supernatural"? He thinks it over for a moment, and-- he can't feel anything in particular from any of those people. Because they're not alive? No, he...
His senses are oddly dulled. Another point for Ciel. While the young master checks around the corner for the coast to be clear, Sebastian takes a moment to do something very unlike himself: actually consider if he's feeling all right. Does he feel strange in any other way?
...Huh.
With one arm still claimed by Ciel, Sebastian sinks down on one knee. No, he's not suddenly fainting:]
What you are saying is that Undertaker is a Reaper? [Please explain this plot twist while Sebastian is down here on the ground. Doing things, apparently.]