I imagine this show will be gigantic in scope even if it's terrestrial based. In the True Detective shows typically they do focus in one location only, whether it be Louisiana, Alaska, etc, but here, the equivalent to that in Lanterns terms would be the whole Earth as that is the precinct they're investigating. It would not surprise me if throughout the show we end up seeing Hal and Jon visiting a wide variety of cities in the DCU as well as countries and even seeing places like Themyscira as they did say that they're exploring an ancient terrestrial mystery.