Make sure you go to your services and check if there's anything that could be downloading or uploading the data enabled. If you're not sure - go to Status > Services and take a screenshot and post it here.
After that, go to System > Administration > RMS. Sometimes RMS communication, even when not used could eat up a lot of data. Try disabling it.
You can visually see from where-to-where traffic goes, and maybe you could find where is data being used. To check that navigate to Status -> Real time data -> Connections
Or you can get a tcpdump file from the router when you notice data consumption. From tcpdump file we can see what could potentially generate this kind of data. You can get the tcp dump file in System > Administration > Troubleshoot. TCP dump is a program used to capture packets moving through network interfaces. By default, the device does not store TCP dump information. You must enable TCP dump and save the changes before you can download the file.
Also, I if you are using an M2M SIM card, which means some data is used to keep the connection to the operator and monitoring.