Hissing at him. Definitely wasn't the action of an innocent. Jack was immediately torn between trying to shoot the offender and the fact that he didn't know whether Ianto was alive or dead at that moment. And the other was too close to Ianto for his tastes. Way too close.

Jack edged closer to Ianto, not taking his eyes off of the intruder as he did, not until he bent down and put a hand to Ianto's neck to feel for a pulse. He breathed a sigh of relief, presenting a moment of weakness because he'd been so scared that that had been it, that he'd lost him again just like that...