From my understanding, these systems send voice to servers to recognize queries, but they detect the prompting signal ("OK Google" or "Hey, Siri") entirely on t

From my understanding, these systems send voice to servers to recognize queries, but they detect the prompting signal ("OK Google" or "Hey, Siri") entirely on the device, and both systems make a "Listening now" sound when they switch from one mode to the other. So, presuming that software is working as described, when you speak near an Apple/Android phone, your speech isn't being sent to the cloud in most cases. Among other things, running the radios all the time to send audio to the cloud would be a battery-killer.