Local forces have kindly shot down a UFO for me.
Just as my Chinook is approaching to land on the mission, a UFO appears above the site. It's just like the terror missions. The shot down alien head icon remains.
I complete the ground mission. UFO breach is part of the mission.
The airborne UFO is still on the map and begins to move off.
It was a Light Scout Sebillian mission, so I would not have expected it to behave as a terror mission.