I have some help from the experts.
There are two possible causes for calls/text being announced when you are not in a moving vehicle:
- Drive detection falses. If you have the Talk-to-Me options enabled in Assist, check is to see if the drive mode detection notification (the icon that looks like a road) is in your notification bar. If it is and you're not driving, "swipe down" on the notification curtain. Swipe down again on the notification item itself. You'll see the "I'm not driving" option. Select that and the device will get out of drive mode.
- We're aware of a rare condition when calls and text will be announced even when you can't see the "drive mode" notification on the status curtain. We're fixing this in the next upgrade. (The simplest way to get out of this situation -- and it's not that simple -- is to leave Assist's drive mode settings on, get in a vehicle, allow drive mode to kick in again. When you do that, Assist will get back to the correct state. After the issue is fixed, you should rarely encounter the issue again. Yeah, I know. Not something anyone is eager to do but we wanted to put it out there.)
The next software release will also reduce the those false drive detections (first cause above), and as I said, it should fix the second one.