Tony's head was still turned to watch her go for seconds after her departure, lips pursed and eyes unfocused. She was welcome to try to play innocent (not the proper term. Correct: not harmful, review, not physically harmful, recommend further review) with anyone else on board, and it wasn't going to be Tony that stepped in. Nick obviously thought she was worth the risk, and he could contain her havoc himself. It was Steve that Tony knew she had some personal discord with, whether she enacted it purposefully or not, and it was Steve that Tony cared about.
To that end, he needed to keep his distance, too. He spared his friend just a sidelong glance, reaching out to touch the door frame again and pausing just long enough to leave him with a nod of finality before turning to find that seclusion he had just been waiting to be allowed.