Why do missions appear and then suddenly disappear?

• When a mission is reserved by an Eye, it will disappear from the map (since it can't be booked anymore).
• If mission was approved, it will be no longer available. 
• If this Eye decides to abandon the mission or the mission is rejected, it will become available again and thus visible to all Eyes.
• Besides this, for some campaigns there's a system in place that switches off all missions once the target number of performed missions is reached (quota reached)
• There is a certain timeframe for missions
  Close to the end date, you will see a message with the end date and time. It means that on the next day (or date indicated) the remaining missions for this campaign will disappear from the map.