I cracked this nut! ... I think. I need some people to help out and see if they can reproduce it in the same way.
My theory is the hissing/interference sound heard from the passenger side speaker under the glovebox is
caused when the 12V battery is being charged.
I came up with this theory while setting up to perform this test of when the 12V outlet has power after the car is shut off:
How and when does the 12v battery get charged?
I was in the car, plugged in a 12V adapter and was charging an iPad. I then unplugged the device and coincidentally started noticing the hissing coming from under the glovebox. It went away after a bit. I plugged the 12V adapter back in for a bit, then unplugged it again. Sound came back.
So my theory is that the 12V got drained a bit from charging, and after unplugging that triggered the car to decide to top up the 12V.
Anyone care to try to confirm?
It's going to be really tricky to try to get service centre to ever reproduce this, but if you can easily do this by charging a device via the 12V outlet and unplugging then maybe we'll help them reproduce it and come up with a proper fix.
Or have I missed some posts and there is a proper fix? For now I've just covered the speaker with a post-it note and that's enough to not really notice it when it happens. It is still annoying though