caba:
Interesting point, although perhaps not for the reason foremost on your mind.
Assuming you speak/read German, you might wish to read recently released (26 June 2002) Report No. 1744 of the Swiss Büro für Flugunfalluntersuchung which - among other items - points to the existence of screens at Skyguide from which planes have simply disappeared in the past (fortunately without any tragic consequences).