Problem really makes me angry.
Deleted ALL Rendering FIles from Event (not only the UNUSED). Quit FCP X. Removed the application altogether. Deleted Caches and Preference Files. Ran PrefsManager from Digital Rebellion to remove other Prefs files (like Motion).
Downloaded a fresh version off the Mac AppStore. Opened my last used Library with Events and Projects. Rendered everything (always keep an eye on your embedded compound clips - they do not render automatically when you hit Shift-Ctrl-R in your timeline).
Quit FCP X and reopened. Guess what? Approx. 20% of the timeline show that orange bar. And - no - it doesn't update when you mouse over the corresponding clips in the timeline. Did a full render again. Quit and reopen. Again - parts of the timeline show the orange bar. Interestingly enaugh: they are different parts from what had been indicated before.
Don't know what to do. Keep FCP X open all the time? Doesn't sound like a solution to me and my MacBook (MBP Retina, 16 GB, 768 GB SSD, Mavericks 10.9.1, FCP X 10.1.1). Performed all the usual procedures like repairing rights, resetting PRAM, deleting Caches, even boot-up in Safe Mode - to no avail.