Yup! Great question! I think it must use the Cross Connect and my phone for this feature. The GPS can track my speed, and the navigation app knows the speed limit.
I picked up the car from the dealer on Saturday of last week, and noticed on Sunday that the car was beeping, and that it seemed to be when I passed the speed limit. I noticed it again last night and tested it at length, driving several different roads with varying speed limits. Always it beeped at exactly 6 miles over the speed limit, no matter what the road and speed limit. I spent a long time testing it. I really am quite certain.
Then I returned home and spent an hour or two looking over the manual, and the cross-connect app guide, trying to determine how I set this feature. I spent some time fiddling in the car as well. I never found it. Or I didn't think I did. Then today, driving to my office, it wasn't happening! Somehow I turned it off. (This car has a sense of humor, it seems.)
I need to spend looking into this more. I'm frustrated. I seem to recall setting something to make this happen, and it definitely was beeping as I was speeding. I don't recall changing anything last night that would be likely to turn this feature off.
As for your car, I very much doubt this was your issue, unless you are using the Cross-Connect App for GPS and music. My regrets for the false alarm.
If anyone with a 2017 understands how this works, please reply!