This is not what so even uncommon, it´s indeed quite normal! However, usually, in the introductions, they indeed tell you who that person is and his/her purpose in that interview.
This person was not there for you, but for the preson who was interviewing you.
In order to become and interviewer -for which in McKinsey you need to be at least at associate, btw-, you need to both:
Hence, message of calm here, this does not affect what so ever your interview, it´s part of the capability building process for the interviewer.
Furthermore, don´t assume that the person was auditing your interviewer, since it can be the other way arround: a seasoned interviewer being shadowed by someone who is just starting.
Hope it helps!