He most likely meant that the concentration of the coolant was 30%. The minimum VW recommended is 40%. Did you top up the coolant or did your mechanic topped up using water only?
I would recommend that you get the water pump checked and cooling system pressure tested. Coolant leaking from the water pump would leave the trail of pink/red residue on the engine, visible from under the neath. If you don't find the leak, than drain 1.5L from the cooling system and replace it with 1.5L VAG coolant (available from your dealer).
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