Do you happen to have a satellite TV receiver connected to HughesNet?
The best thing to do to figure out what's utilizing the data is to not just turn the things off, but disconnect them all together, then add them back in one by one until you find what it is that is utilizing all of the data.
I suspect this is one of the most dishonest companies out there. I’ve been told I must pay a fee to stop my service even though the speeds are so awe full I cannot watch a simple video clip or even use their search feature in their help section, when I check the Speedtest it says it’s excellent. No way to prove how my data is being drained, even though my family was traveling not long ago they said i used all my data for the month in three days. Was forced to keep another service provider just to keep my kids able to do their homework. I’m at the highest subscription level paying nearly 90.00 per month and the speeds are awe full even though their testers continues to read “excellent”. They should be reported to the better business bureau but it’s probably not worth the hassle.