It's where their parking spot is?-Could be the answer.
Are you certain that it doesn't fly between your sightings?It is Xmas,lots of freighter types have numerous extra legs this jolly time of year and utilise overflow parking areas which are usually located in the more unfashionable parts of the airport.
If it doesn't move in the next month or so then creep up,put a salvage sticker on it and it's yours!