I think that Vienna is a destination and they have to find a clue box at the Weihnachtsmarkt or have a task there (find the right stand, drink a cup of Gluehwein and eat 10 Lebkuchen), then on to the next task. From Austria they could pretty much go anywhere in Europe! It's all way too close to speculate. The Madrid sighting to me looks like a trip to Elimination Station or a connection from Africa to Austria. The cheap airlines in Europe like Ryan Air usually don't use the main airports though, so I would eliminate that possibility.