Help me find an old episode.

The plot is something like a man gets killed and police can't find the killer as there wasn't anyone else in the room except for some mannequins in the backstage of a theater. And then turns out that one of the mannequins was actually a real person. It was an ex student who has a unique talent where she can pose as a mannequin for extended period of time. And she was there and she killed him.

All facts may not be correct as i have seen this 13/14 years ago.