2.0 ALT Untraceable use of coolant?

[VolksKid83]

I came home from work today to have my dad complaining his Passat is unusable as it is excessively using coolant! Apparently after topping it up to the max, he drove three miles to have the 'Low Coolant' warning appear, and so on from there.

Now i have firstly checked for the obvious, it leaking. The radiator is sound and dry, i have ended up jacking it up, dropping the under tray and having a good look around tonight in the rain, there's no sign of coolant leaking from anywhere. We do run a strong mix in our cars so the pink will show when it leaks!
The only evidence of coolant was along the front cross member, i traced this back up to the expansion bottle, which would fit the fact he said he spilt some whilst topping it up!
Being thorough i released the expansion bottle and had a good look for splits, and i found nothing.

There is no Mayo in the oil, and no oil in the coolant. I had the car running on the spot for about 20 mins, i could see no leaks, and there was no steam pouring from the exhaust, it all looked good for me.

The front carpets are dry, and there is no smell of coolant inside the car.

So where is the coolant going? Anyone have any bright ideas? I'm stumped if i'm honest!

The car is an 2003 2.0 Petrol (ALT code).

Cheers guys!