How do I make my iPad Retina wifi hotspot discoverable (tethering 3G) when off and unplugged like my iPhone 6S?
You'd have to be on the Personal Hotspot menu from Settings. This "forces" the broadcast and allows devices to connect while you're on this menu.

AFAIK I'm afraid you can't make it discoverable while it's off, though I think there is some sort of timeout after leaving the Personal Hotspot settings that allow connections from other devices.