I have the exact same issue. Have even paused all devices, turned them off, etc. for hours and the app showed very high usage. Talked to tech support yesterday, they even called their help desk and no one knows what is going on. Even checked today, app shows my 4 profiles all with 0%, but the graph indicates high usage. The site shows two profiles with 1% and the graph shows high usage. Restarting the modem will resolve the high usage in the graph for 12 - 24 hours, but then it just starts again.
Doesn't seem to affect my actual data usage so I'm just going to consider the xFi app useless (pretty typical for Comcast tech) and not worry about it. Just waiting for the day that another company offers very high speed Internet. Comcast goes away immediately.
I think my Chromecast Ultra is "phoneing home" over its ethernet connection even though I haven't watched anything for over a month.
That would appear to be the possibility that my data usage goes up faster than I was aware.
I am having the same issue. This makes parental controls on XFI USELESS just like Comcast. I pay for pro speed and some days barely get any but they can't help figure it out either.