Just a thought...
I replaced the radiator on my brothers mk5 a few weeks back and that had a few leaks that weren't noticeable when it was on the car.
There were small pin hole size leaks in the rad core on the front side that you can't see as the bumper is in the way.
Also a leak from the lower rad pipe, bottom right side as you look at the car, replaced the o ring inside the plastic connector to sort that.
May not be your issue, but worth mentioning.
Jas