if the leak is sufficient, it could be enough to activate the boiler to come on briefly on dhw mode. Best thing to do is get a gas safe eng round to have a look, as it can be quite hard to say online. I'm not aware of any particular faults on the greenstar that can cause those symptoms.
I know there was a minor issue with the gas valves not being set up correctly at the factory end last year, but it wasn't anything dangerous, more a nuisance fault then anything.
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