If you're on a mac, go to Sound in System Preferences. Click Input. Do you see activity in the input monitor? That means your microphone has been on the entire time you've used your computer and has heard everything you said. I HIGHLY recommend disabling your internal mic. So does Apple.
http://images.apple.com/support/security/guides/docs/Leopard...
Is there a more recent version of that guide (e.g. for Sierra)?
Not sure, haven't come across it. Here's a pretty paranoid setup, can't totally vouch for it though https://github.com/drduh/macOS-Security-and-Privacy-Guide