If i was suspecting HGF - albeit on my 1.8t - i would be looking for the following
1. Brown mayo on the dipstick
2. Brown mayo under the oil filler cap
3. Pressure escaping from a cold system - ie after a good thrash - say 10-15 miles at motorway speed - leave till next day and open expansion tank cap - if there is a loud sound of escaping gas then that points to exhaust fumes in the cooling system and points toward HGF
Did your mechanic do the chemical CO2 test on the coolant - not sure if it gives false positives - but it sems to be in widespread use to identify exhaust gases in the coolant.