As reported in Bloomberg <https://www.bloomberg.com/news/articles/2019-04-10/is-anyone-listening-to-you-on-alexa-a-global-team-reviews-audio>, Amazon has thousands of staff listening to what their devices hear. Based on the information provided by Amazon, it is clear that their devices are listening at times when the user hasn’t directed it to with an “Alexa” preface.
This has been the assumption within the IT security community ever since voice-response devices hit the market. I have long found the behavior of Apple’s Siri to be suspect. For example, I may provide Siri with a full paragraph of spoken content, and then watch as Siri enters text, removes some text, enters some more text, edits text, and then completes the paragraph. This is not the action of AI, but of a human translator.
In the case of Siri, it can be disabled on both iOS and macOS devices. It is different with Amazon Echo devices. Without voice response, they serve little purpose or value.
For me, personally, I’m leaving my Echo devices (8?!) unplugged until needed.