Chris,
That's the point, there has been no external evidence at all of an obvious/ visible leak. The car is garaged at night and it has a smooth, clean floor that is always bone dry each time the car is moved.
I simply noticed that the coolant level had dropped between the range fill markers on the coolant expansion tank (during my regular fluid/ checks) and so - being diligent - went to my local VW Parts Dept, bought approved coolant and topped it up immediately (not much top up needed at all).
This morning, before setting off to the further away servicing dealership, I checked the coolant level and noticed it had dropped again but not below the minimum level. When arriving for the service, I asked them to please look into the issue and they came back after the service to say it needed a new radiator because it was leaking.
However, when I questioned where it was actually leaking from, they couldn't be specific citing that until stripping things down, they wouldn't know. My question then was well how can you be so specific that it does actually need a new radiator?