So I tried something. I disabled WLAN permanently (and only activate it if it is really needed) and removed my microSD card.
THAT did the trick! Don't know which one or if both help such a lot.
I had some mp3s for ring/sms tones on that card (in /media/notification) and Android silently moved apps to that SD (not the internal flash).
That means that Android somehow always needed to access the card (->Android OS), for playing tones (sms) or ringtones. Or even for loading icons in the menu from the SD.
When I removed the microSD card my battery lifetime instantly went up from 12-18h (Event / Suspend / Android OS Bug) to 3 days!