Bung:
From Reddit
"The current theory is that it has to do with moisture buildup inside the ear cups causing a grounding/short somewhere in the ANC circuit which somehow causes microphone feedback."
The repair shop probably just left them on the bench so there wouldn't be the same humidity build up you'd get wearing them.
That's precisely the issue, and is what I passed on to both Sony corporate and Infinet (the repair agent) several times. I said they would need to be wearing them for at least an hour before the issue arises, which I'm guessing people in a repair centre don't really have time to do.
If I don't have any joy with the retailer then I guess I'll have my first CGA/disputes tribunal experience but I'm hoping to get it resolved before then.
Bung: Shame that Airheads didn't pass the focus group :-)
At $999 a pair, I'm thinking Airheads is what we'll probably call people who buy these (myself included).