If it had a 100,000km service they will have replaced the timing belt and radiator coolant, among other things.
Would suggest it hasn't quite been bled properly and still has some air trapped in the system.
You could keep an eye on it and top the header tank up as the air slowly bleeds out or take it back to the mechanic.

Personally I would get the air out quickly and get the coolant in. The next few days are going to be very hot.

Cheers
Ken

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