The app data usage section of the Settings mentions that "Android OS" has taken transferred 1.73 MB of data in the background.
What is that caused by? I have disabled captive portal detection on my device. I'm running CM 13.0 without Play Services.
I've noticed on my CM13 phone, I will periodically get notifications when there are new nightlies out. Have you tried disabling the automatic checks for updates? Simply go to Settings->About phone->CyanogenMod updates->Check for updates-> Set to 'manual'.