He instantly notices Sam's step back. It makes sense, at first. Last thing Dean remembers is looming over his baby brother, ready to kill him. And that was after beating the crap out of him... so yeah, not surprised at that.

He slows down, about to throw the scythe away and explain that he's not gonna hurt him, but... It's not right, what Sam asks him. That's not the question Sam would have asked if they had been snatched together. 'What happened?' or 'where are we?' or any variation of those two and Dean wouldn't have suddenly stopped dead in his tracks, but that question there... that makes him suspicious.

Something's wrong.

"Sam... " If that's really you "Do you know where we are?"