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.

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.

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.

No.