Go out of the building that has the sound stage in it the same way that you came in. When you get outside turn left. There is a door that is painted to look like an alley. Investigate the door, and Cole will kick it in, revealing the door to the secret room on the other side of the mirror. Follow this guide for more info: http://www.sleeandtopher.com/guide/l-a-noire-the-fallen-idol-case-guide/