Jack simply watched from where he'd settled himself, in a relatively dark corner. All he could see were shambles of what had been his office, with Ianto standing near his desk, and the woman he didn't recognize at the door. He watched, and waited, and hoped at the moment that neither paid him any attention.
On the other hand, he could cross this out as a place to secure the Master, once he found one of the variations of the Time Lord that was running about.