"Magic" internet usage

Hi! Could you please help me understand how it's possible. Today first day of December, now it's morning at PNW and I've checked that this night was used more than 400 Gb. It's really weird, as all my devices was in idle, was no downloads in my devices - just iPad, MacBooks, iPhones and apple TV, nothing else connected to WiFi. No software or OS download was scheduled. The same was yesterday at night, when somehow at night it was used more than 800 Gb, and now I should pay extra $100 for that. 

In Xfinity app I don't see new devices (so I assume no neighbors in my WiFi network) and there was no notifications that new devices connected.

Xfinity online chat guy can't help me, only try to sell me Unlimited data plan, that I don't want. Do somebody know what can happen? Thanks in advance!