Well, well well. So Illustrator crashed (stupid me did believe adobe saying it was more stable after the update some days ago) and after killing via task manager and re-opening it only did show one of the three opened documents. Unfortunately it was only one file I needed to copy and paste an element from and it was not important.

The file I was working on originated in an One Drive folder. Maybe there´s the issue. There should have been any temp-file somewhere, though.
Autosave is on and saving every 2 minutes, default.
Not in the users files nor in the file folder the recovery file was to be found. It was nonexistent like a ghost.

Stupid me relying on autosave and recovery, which worked quite ok in Indesign the last months. I will go back, forget about those 2018 functions, back to my workflow of 2008, when you had to panically save after every click. yay, I am so happy. Past, here I come! I missed you! I got too comfy, treating manually saving as an option only, and was fooled.

Everything up to date. Win10 / Illu 24.0.2

Perhaps you might implement a kind of warning if an autosave file could not be created, or something like that.
Maybe recovery files could be kept a little longer and not be automatically deleted after re-opening Illustrator once (if that is what is happening).