A bit like the scroats do to braek in but in reverse 
I would whole heartedly agree chap sounds likesome panel damage to either teh door, or one or other of the surrounding panels to me, but could be as simple as a rubber door trim, seal not being seated correctly.......although the 500mls bit would kind of suggest it had been lying unused for quite sometime Vw owning it and doing 10k in a short period i would hazard a guess at it was either a hard worked demo which i doubt or a lease/rental our local chain Arnold Clark i know keeps his till they are 12k and sells them on through 20-30 makes he now sells it is apparently cost effective as no repairs other than under warranty.....which means its done a large mileage then nothing i doubt very much i would of laid very long to sell if that was the case so why it only then had 500mls i cant answer other than if it was sold as a cat d and was awaiting repair possibly.....
A wee tip for you take your chassis and reg no and go to another Vw garage in your area, who should be able to tell you a full rundown on what has or hasnt been done whilst in Vw ownership and beyond, service, repair and re-call wise and a simpem £3 text check will tell you if its ever had accident damage if nothing else it will ease your mind.
Just don't do what did with my last one [T4] where a buyer fully checked it out and found out a lot i didnt know almost costing me my sale, DO NOT only use nor totally trust HPI etc as they also get it wrong......
