When clicking on the round icon with the iron cross in it, a box appears that has a white bar at the bottom, when that is full you have fulfilled the mission. But it will be sometime later that the radio message to cease all operations and return to base will come.
My return order didn't come till I had just almost reached the second waypoint you see in this shot over to the right on the map, which also shows the mission icon opened up and the white bar full.
