Thanks for reminding me of this. This is looking less and less malicious at this point as `10.0.0.3` is my phone (using AT&T, which is where all the traffic was destined).
I can't explain this part yet. I was asleep when this happened, so I wasn't even using my phone. I may be wrong about 300Mbps being to the AT&T. public IP, as my router shows a much lower rate. That might have just been the total traffic I was seeing internally on my private network from multicast.
Do you mind sharing what gets you that kind of data usage? Just hours of FaceTime calling or something else?