I'm using Ubuntu 16.04 and Gnome. I use the system monitor widget and noted that network usage was unusually high both for downloading and uploading. I find this question and I installed
nethogs to check what happened and found that network usage for the process
/usr/lib/evolution/evolution-calendar-factory-subprocess was pretty high.
In the online accounts window I turned off and on the calendar item, and after that the process came reduced network usage.
Could it be a known bug? Was it sending/downloading data without consentment?