Apu bleed
The air from a ground airco unit goes directly to the mixing unit so packs are not involved and as there is no bleed air.
You can check if ground air is connected by observing duct temps on ecam. Although sometimes the units dont work well so you still dont know if the airco is on. Then switch off cabin fans and check if you feel any air from the "eye ball" if not then you know there is no air provided and you can stick on apu bleed. (Of course provided you switched apu bleed off before)
Not really sure why airbus could not create an indication on ecam.