>newer Alexa devices have "presence detection"

Not even the biggest privacy issue of using Alexa devices. I think listening you 24/7 is a bigger potential issue.

Not sure if Alexa has this, but cheap mm-wave wideband multi-GHz sensors(or radars more accurately) now enable more finely grained human presence detection and also human fall detection[1] with the right algos, so you can for example detect if grandma in the nursing home fell down and didn't get back up, but in a privacy focused way that doesn't resort to microphones or cameras. Neat.

>Reminds me of the story about how the Google Nest smoke detector had a microphone in it.

Vapes have microphone arrays in them to detect when you're sucking and light up the heating element. Cheap electronics have enabled a new world of crazy.

[1] https://www.seeedstudio.com/MR60FDA2-60GHz-mmWave-Sensor-Fal...