M1 MacBook Air using large, inconsistent amounts of network data

I was recently checking the Activity monitor for my Eero Wifi network here at home, and noticed something startling: on some weeks, my network throughput was just insane.

As this chart shows, my MBA was responsible for over 500GB of downloads in one week (!!), and here's the rub: it wasn't even being used. I was away that week and left the MacBook Air at home, doing nothing. Has Power Nap gone awry?

Other weeks vary, but the MBA (SilverSpoon) is the biggest hog in the house by far. I am NOT using it to download huge files, nor am I using it to watch copious amounts of video. I have ordinary cloud services installed (Dropbox, iCloud, OneDrive), but I'm not using them for large amounts of anything. I use iCloud photos, but these amounts are greater than my total library several times over. I am NOT using any cloud backup services, just a local Time Machine drive. I use my MBA for email and writing, primarily.

The other 2 Macs in the house use a tiny fraction of this amount, as do the iPhones and AppleTVs. I could easily blow through my Xfinity data cap without doing a thing.

I have just installed Little Snitch to see what I can learn. Has anyone else encountered this issue??