Does Google listen in on your life?

Many Google users believe the company records audio through phones and computers, then applies those recordings to the AutoComplete options in Google Search.
I asked followers on Google+ about this and one wrote: "If me and my wife are talking about some random subject like how long turtles live, I'll type 'how' and it will fill in 'long do turtles live' like it was listening. And I'm terrified and impressed."
The anecdotal evidence is convincing. Some say AutoComplete is more than just specifi