How do I make my iPad Retina wifi hotspot discoverable (tethering 3G) when off and unplugged like my iPhone 6S? I can use my Ipad Retina (4th Gen, iOS 9.3.1) when plugged in via USB to tether the 3G connection. 

After it is plugged into USB, then I can see it shareable via wifi from other devices.  (My Mac which is it plugged into via USB, and my iPhone 6S). 

After I unplug the USB from the Macbook, the devices that connected to the iPad via wifi can still maintain the connection, but it is no longer discoverable on wifi. (And if I restart the iPhone it remains paired). 

My question is: 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.