For not being dead, your scent is unnaturally ethereal, beyond the metal.

[He's just saying bro. You smell like a ghost.]

Or do you expect me to believe that you are not possessing that metal construct?

[He has no idea how hard he's calling Mettaton out, actually? He just knows what his sense of smell is telling him, and as a supernatural creature himself, he's not going to sugarcoat it.]