Why in the world would Honda not include an extra ground cable when the duel battery system is so popular ??
Honda didn't install it for a dual battery.
They installed it for a factory-installed winch running a single battery.
ALL winches package come with ground wires, so there was no need for it.
In my opinion, it's best to ground back to the second battery anyways when installing a second battery.
This keeps your flow confined between the 2 batteries and not through the frame.