Mine ended up having a cracked head, just like my mate's A3, and was replaced under warranty - which nI chose to extend.
I would check for obvious leaks first, as this costs nothing. Maybe some sort of sheet under the car at night. Then, if nothing appears, some sort of radiator sealant is the next cheapest option.
Someone please correct me if I'm wrong here, but if there is a crack in the head, you wouldn't see any obvious leak, as it would just disappear? Steam?!