This is not strictly true. I examined a master's degree thesis last semester where the student proved conclusively that Alexa devices are periodically spamming bursts of data back to Amazon HQ even when not woken. Beyond a heartbeat, too - MBs of data, not KBs.

No clear idea what that data is, though. There's at least a small chance it isn't benign.

Amazon is surely aware of the aftermath of Google's accidental Wireless network war-driving.

While absence is hard to prove, an intentional or unintentional data collection beyond what is stated in their policies is highly unlikely.

I also don't think any insiders at any of these companies have leaked that they passively listen to what people say.

Do sample recordings, reduce bitrate (you can have good voice recording and playback at less than 3kb/sec) and upload only if the device detects something worthy of notification.

Hell, don't even send the sound, just send a weekly report of events.

No.