Have you created a work center for the pick point? From there you trigger the return of the HU. I have never found a way to automize that (yet).
Define the work center in customizing. In there you find a field for the return process type, you can also define a return storage type and section. When you log into the work center, there is just a special icon.
The documentation is a little bit confussing, i agrre. It refers to in the first part that the HU, as it is not full anymore, is now stored in a different storage type. But why this should be with fixed storage, is not clear to me.
brgds
Juergen