Is there a way to make a plasma panel only autohide when there is NO application running? And only then?

Not at present (as far as I know). Would you please elaborate on why you want this behavior?
The common use for auto-hide is to get out of the way of open applications, but then to be present and available when there is nothing open so that you can use it to open things.

I use a top-panel with global menu in it. When there is no active application I have no need for the global menu. And I like a clean desktop when I van het it.

One idea for that would be to set the panel to collapse to the contents. Then it effectively goes away when nothing is open

That would bring me in the right direction... But there are also "plasma-addons" aren't there? Could there be a solution in that corner?

You can certainly browse the selection on store.kde.org to see if there's another panel you can install.
I think right now the selection is limited to the default plasma panel and Latte Dock, the later of which offers additional settings and features but I think doesn't have this feature (yet).

I use latte-dock only at this moment. It is great.
Thank you!