Steve,

Because this is, according to David, a "downstream panel", it would be connected with an equipment grounding conductor, not to the grounding electrode conductor, water pipe or ground rod. Yes, the 'equipment grounding conductor' does need to go back to the service equipment, and thus back to the grounding electrode conductor, etc., it will not be connected to them, as is my take on what you said.

There just is not enough of the panel in that photo to show how this is "grounded".