I work at Google, and have no idea where you came up with your claims.
i hear from acquaintances who work in Google that the algorithms they run on emails do something much like this. among other things, they know when you are thinking of taking another job almost before you do.
While I cannot disprove that HR is running sentiment analysis, we have company-wide surveys every year that they could use, biannual reviews by co-workers, and quarterly short reviews from managers. All of those probably have much higher signal/noise ratio than rummaging through peoples' email. Also, they type of people who can do that kind of NLP are probably better off working on NLP-related areas that help the company such as Android, Search, or Ads.
word is, among the things you must not say on the phone inside the pure-freedom, do-no-evil world of Google, is "let's take this offline" or anything else indicating you don't want to talk about something on the phone, since that's an instant tip that you want to say something unsurveilled. coming soon to our entire society!
This is not at all true. First of all, for internal communications hardly anyone uses phones anymore -- chat, voice chat, and hangouts are a simpler and faster options. The only people frequently on the phone are those talking to external people (sales folks, customer reps, etc). I guess those "calls could be monitored for quality" but that has little to do with the panopticon-like claim you are making.
Also, in the context of meetings, people say the phrase "let's take this offline" all the time, indicating that they don't want to start some (possibly long) side discussion in a meeting with multiple participants.