Some time ago some people in Vienna started to create ghostbikes too. Sadly, town officials remove them regularly. In the beginning they used notes as Anna described already. I know of some persons who regularly removed this notes from ghostbikes. Recently ghost bikes were removed over night, without having a note sticked to it before.
Alec Hager from IG Fahrrad (a Vienna based bicycle advocacy group) now started a pedition. He demands that the ghost bikes should be left where they were installed.