I was playing around with some shadow passes for a project. The walker is backlit by a single sun light. When the raytracing is turned off the mesh is very dark, especially the front of it. But when raytraced shadows are used the entire object lightens up, simulating ambient light. No changes were made to the lighting setup.
Are you sure that AO is disabled? I think I can even see the AO grain in the image…?! If that’s not the case, I’m not sure either.
Anyway, also wanted to add that is quite a nice model, judging from the rather small pic