OK, I have solved (my) disappearing data dilemma. I hope it helps someone else. I found the answer a couple of months ago and things have been back to normal ever since I unplugged my Google ChromeCast dongles (2 of them).
Yes, (and I hope I'm not repeating myself
) Chromecast downloads those beautiful backdrops to the tune of 8-10MB an hour. I have two of them so it was totaling 15 to 17MB/hr. If you add that up it's over 10GB/month but you have to consider the bonus time where the data used is not going against you. Since I've unplugged the ChromeCast dongles, data usage has been normal.
Most people won't notice the data usage but satellite people sure will! As of right now, I know of no way to stop Google from downloading this unwanted data. Too bad because I really enjoyed my ChromeCast.