A quick search throws up this as one of the most recent investigations (and seemingly quite a thorough one):
https://www.iot-tests.org/2017/06/careless-whisper-does-amazon-echo-send-data-in-silent-mode/
Seems inconclusive.
A quick search throws up this as one of the most recent investigations (and seemingly quite a thorough one):
https://www.iot-tests.org/2017/06/careless-whisper-does-amazon-echo-send-data-in-silent-mode/
Seems inconclusive.