Even if you did not know in advance, the student should have had your name in advance. Did she know your name when you were undergrads? Do you have a common first and/or last name? Did you hang out together with a small group of people on a regular basis or was she just a recognizable face in the crowd on campus. Was she in a leadership position or a tutor that you recognize on that basis?
It could be that she did not recognize your name when she saw it on the list. It may be that she recognized the name but did not think that she knew you well enough to make it an issue. It may be that she would be biased in favor of your admission and decided to keep quiet.
The time to speak up was when you recognized her (or recognized each other). At that point, there may have still been time to swap you with another interviewer. Unless schools are doing interviews before sunrise, this interview was yesterday or longer ago. I'm feeling like that ship has sailed but maybe other adcoms think that it is not too late to notify the admissions office. You do run the risk of making it appear that you are now because you have doubts about how well things went in the interview.