Lost Chao Locations
Locating the Ancient Ruins:
After the elevator part, you will get to the second checkpoint and then through a tunnel with a few enemies. In the next section there will be a platform that rises up off to the right hand side. You can use this to get on the roof of the structure you usually walk through. Once here, if you rotate the camera you will be able to see the Ancient Ruins on another platform. Jump and hover over to reach it.
Finding the Lost Chao:
Activate the ruins with the Mystic Melody and a spring will appear on top of it. Use the spring and you will reach an area with lots of G.U.N. Beetle robots. Destroy these and the gate will open to the barred area where the Lost Chao is.