"Say what you will, but you have to admit that the Lucians are code masters. Who else could hack into an airport computer and change the departures board to create a secret message? They must have been sending their agent somewhere pretty important to go to all that trouble. But why was he spotted carrying a book about Nikola Tesla? Not exactly light plane reading..."
Question: What is secret of Location X?
In the mission The Mad Scientist's Clue, the answer to the departure board puzzle is Croatia.