I would like to achieve the following workflow:
When I open files to edit and close emacs, I want this changed files to be temporarily saved, so that I can very quickly close Emacs / move to other work items. The next time I open Emacs, these "scratch-items" shall be recovered.
I am aware of auto-save, but I want Emacs to "silently" open the auto-save files instead of prompting me what to do for every buffer.
recentf-mode
is what you want, but it's not clear from your question. (It does not save the unsaved state, however. It just gives you quick access to your last-used files.) See also the Emacs manual, node Auto Save.