Hi there,
My Gti was experiencing the exact same problem, i took it to one dealer and they did full pressure test and found no issues, it then happened twice again with a puddle under the car. I think it might have happened after some fairly full on driving so maybe is caused when things get really hot.
In any case, took it to Drift Bridge in Epsom who were excellent, they found a leak from a hose connection to the head and replaced it all under the used warranty (thankfully as the invoice came to £270, £200 of labour

)
Hopefully it will be fixed now, I have a bottle of coolant in the boot if not, seeing the number of posts on this site about this there is clearly a design fault.
Anyway, hopefully yours gets sorted too, get them to check the pipes to the head,
Cheers,
R