it could be a few things...could be the head gaskets weeping at higher boost, so easy check to do:
with the car cold open the head tank is there any pressure in it? (there should be non if there is thats not a good sign but we will come back to that..)
tighten the lid back up and start the car
give it a min to idle tn give it some revs to get the car on boost, get the revs up past 2k and hold for a min or so, but dont hold it there long enough that the car starts to come up to temp.
return to idle,
open the header tank, is there pressure there already? if so the head gasket is going and pressuring the coolant system which will be forcing water out past a seal on boost, when crusing on the motorway the revs are up and the turbos on full chat hence the use when crusing but no round town.. my old mk4 did this, it would force the coolant out past the water pump, but most of it was sucked into the combustion chamber and sent out the exhaust making the back window very oiley and messy! 1 head gasket change on the drive and an inspection of the head, all was deemed well, rebuilt with new gen parts and then I chipped the car, (that was with 170k on it! it had been using water since about 150....as I waited for the cambelt change to do it) the car is still gong now with its new owner at well past 200k!)
other things to look for are:
sniff the coolant, if its smells a bit diesel like then again head gasket,
look around pipe joins for a pink dryed substance, that shows G12 is leaking out, the water part evaps off but over time it will leave the pink chemicals behind as a tell tail
wet carpets near the centre console? - heat matrix is leaking, another point to this can be windows that steam up alot and sweet chemical smell in the car.
park the car some where clean over night after a long run and look for water drips, a peace of cardboard can help as again that will not allow all the coolant to evap / run away it will leave a mark where its been wet and stained.
