It might not be sending your audio, but to work it does have to constantly listen for that trigger. It doesn't always get it right either. My work phone is a iPhone and Siri triggers all the time.

I drive alot for work and have to setup routes in maps. Just with the radio on I can see Siri triggering and listening multiple times a day. It's weird because it's often a phrase not even close to "hey Siri".