Hey +TSRA. Thanks for your thoughtful reply.
I do have some lingering misunderstanding. Specifically considering the aircraft stationary on the ground in a nil wind situation. I'm aware that in flight the cabin static pressure tends to drop in unpressurised aircraft leading to airspeed errors, supposedly due to suction from the faster air outside (like an inside-out venturi), however wouldn't this effect not apply in the case of a stationary aircraft in a nil wind situation? No airflow outside leading to no difference in static pressure between the main and alternate sources? These are the cirumstances that I do the preflight alternate static check. Surely there is something else as the cause. On a different forum I've since been advised that it may be something to do with the way the alternate static is mechanically designed but no details as of yet. It's not a big deal but I'd be interested to know the why.