Yeah, I know about that. But that doesn't really help if the app only listens while foregrounded. (In that case the red bar does usually show up briefly before the app responds to the backgrounding event, but that's not very noticeable and basically a bug.)

Location Services is a model here: like sound recording it asks for permission first, like sound recording it flashes the menu bar if an app is using precise location in the background, but there's also a small icon that shows up in the menu bar whenever it's being used, even by the foreground app.

Except for system software like Siri. I didn't realize I had the 'Allow "Hey Siri"' option (which constantly listens for the trigger word) turned on until I happened to check the settings, which was a little unnerving.

You're right, I hadn't considered Siri. But, I don't think it would be much use as I'd assume if the system software was about to do something nefarious it would just disable the notifications.

Best assume if you have Hey Siri enabled, it is always listening.

At least Hey Siri is an offline feature, it won't send anything over the network unless it actually detects that phrase. This can be easily verified by turning on Airplane Mode and trying it, it still works.