It's because of Nyx that Jack is here. She's the one who sought him out, digging through his aliases and false trails, tracking him down to recruit him into the struggle. She convinced him to cross lines he had sworn never to cross. It was inevitable that once he was committed, she would be among the first Jack would track down to recruit in turn.

When Jack finds her base, he approaches cautiously, making sure nothing is trailing or tracking him. He's been out of the game a while, and if he makes any slip-ups here it's over for everyone. He doesn't knock until he's sure it's safe. He's older and he has a scruffy, graying beard, but it's still very clearly Jack Bauer at the door of Nyx.