It is always listening. A coworker has Siri enabled and it regularly interrupts meetings because it thought some random noise was a wake word. It then proceeds to do whatever Siri does, which I assume involves sending whatever it hears after that point back to Apple.

I carry around and iPhone where I've done my best to completely disable Siri so I expect it not to be listening. That's antithetical to the use-case of Alexa and similar devices.