I'd go along with the motorway and damp mileage being the cause � 10,000 miles and nothing in the airbox pipe, then 2,600 mile trip with over half covered in pouring rain and around 500 miles of motorways and there was about 10mm in the bottom of the tube. Emptied out and back into the old pattern of 100+ mile a-road blasts and it's all back to normal.