I was suspecting this was webex, I use an external audio device (plantronics/polycom Calisto 7200) that has an indicator when the microphone is active. It's only anecdotal, but for me when the webex ultrasound features is activated, the microphone is constantly active as long as webex chat is running. When the ultrasound detection is disabled, the microphone is disabled when not in a meeting. As an aside, this setting was reset recently, but that just seems like the ongoing dark pattern/incompetence of "forgetting" privacy settings between software releases.