It's easy to forget, now that he's not sleeping in a bed with two cold vampires any more, that Ianto's no longer human. He so rarely lets the demon out - which makes it all the more shocking when he does, and Jack can't stop himself rearing back slightly as Ianto's face changes, into something inhuman, and yet he can still see in those yellow eyes all the pain and anguish of the young man back in Cardiff who'd yelled at him when he wouldn't save his Cyber-girlfriend.
Oh yes, he's hearing those words too, and it's oh, so bitter after all they've gone through.
After that first involuntary recoil, he stands his ground, half-hoping that Ianto will come at him, bring this down to the level he and Spike used to work at in the early days, give him something physical to fight instead of all this bitter iciness.
And then Ianto shifts back and the moment's gone, leaving behind it just a cold emptiness.
"No fucking clue," he says hollowly, and this time he does walk away, with a dramatic swirl of his coat around his ankles, just because.