I recently had a huge data loss problem. I would lose up to 1.2 gb in one day! Strangely it always used any time data, never bonus bytes. The problem turned out to be an I-phone. It seems these things use large amounts of data whether they are on the internet or not. The owner of the phone assured me that she had disconnected from my wifi, and it would go away for a while, but would always reappear in the devices list on the HT2000w modem. I guess they reconnect automatically.
I changed the wifi password and did not give her the new password. Problem solved. BTW this problem only appeared after she "upgraded" her old Samsung to an I-phone.
I see it's your first post here, so welcome to the community! Nice job tracking down the data usage culprit!