macOS Content Caching serving lots of data to unknown clients
Need more details to really say anything conclusively.  When the data transfer is happening, you should run sudo nettop and look at the local addresses the AssetCache service is connecting to.  This will reveal what device on the local network it's serving, or the lack thereof. 

Some other quick thoughts:


I'm fairly sure content caching DOES apply to the local machine.  So updates downloaded by your Mac Mini will be counted and cached.
Do system wide network usage stats show at least as much data transfer as the Content Caching stats? If not then it's probably just a bug in content caching statistics.
Are you using iCloud? Content Caching can include iCloud content if you have that enabled.
Do you have an iOS device plugged in over USB and have tethered caching enabled? That's a potential source.
Try looking at the content caching log while this is happening: sudo log stream --predicate 'subsystem == "com.apple.AssetCache"' --info