sudden high data use

My internet data use suddenly spiked from a many years-long pattern of less than 400 gb/month to more than 1000, and with seven days left in the month I'm pretty certain I'll be over the 1229 gb limit.  I found out when I got a msg from Xfinity last week informing me that I had already 75% of my limit.  Since then I've been monitoring my usage via the usage meter both online and in the Xfinity app, and it continues to creep up.  The usual pattern is maybe 24 hours with normal data use (7-8 gb per day), then all of a sudden in a single hour my usage spikes up by 20 gb or more.  I've also monitored the family members who use our network via the app, and see nothing out of the ordinary.  There has been no change in the pattern of data use in my household, and I'm at a loss to find a cause for this.  In addition, I've been checking for unassigned connected devices and find that every day or so something new shows up with only a number to identify it (e.g., 7E7199D281B8, CA1B48F77E74) - when I see one of these I immediately pause it and have noticed no bad effects from this until today when my wife's iPhone 8 wouldn't connect and the iPhone error msg said it was paused!  Go figure.

I've also done a gateway reset (I'm using Xfinity's XB7 gateway (the white one), but that didn't solve the issue.

I've talked with a service tech at Xfinity, and got no help other than a suggestion to purchase an unlimited data plan.  I'm pretty certain that this problem is on Xfinity's end, and that somehow my data use is being mis-reported.  Can anyone help with this?????