Titan. Arboretum. There is a ghost scannable in the same area when you encounter the first crystal if you turn around and go up the stairs to the back wall. It's the camera one. The eye will follow you and monitor you, but it's also the only door which has an arrow on the ground indicating to go forward. We've exhausted all the dance routines still, but it's no coincidence it's not available during the strike. This is one of those secret areas probably like the Dreadnaught door which led to the knight.