From FreeSpace Wiki
Mission Event Incomplete (Boolean operator) Returns true if the specified event in the this mission is incomplete. This sexpression will only be useful in conjunction with another sexpression likehas-time-elapsed. Used alone, it will return true upon misison startup.Returns a boolean value. Takes 1 argument... 1: Name of the event in the mission.
Don't use this for debriefings. All events that were incomplete at jump-out will turn false.