It depends on what you have connected. Many devices can use data automatically, even when they're off, as some aren't really of, but in standby, and still connected.
I know you said you were an installer, but just in case you're not aware of it, you can see what is currently connected, and what was connected, but is no longer, since the last time the modem was plugged in, here.
Do you possibly have a satellite TV receiver connected to the modem, as those receivers can use a significant amount of data, even when no on demand viewing is taking place, as well as when turned off. Devices with a cloud connection can also chew through data.
only things that are connected are my playstation and a roku... i don't play games on the playstation just use it to stream hulu and the same on the roku. never really thought about what you said till ya said it lol but just watching hulu wouldn't go through that much in two days i wouldn't think but im used to really high speed internet then when i moved up here hughesnet was my only option
I'm not really familiar enough with either one of those devices to know what they could be doing on their own, if anything. With regard to Hulu, I'm not sure how much data is needed. I know with Netflix in HD it's about 3GB per hour, but I think Hulu is supposed to be lower than that.
Hopefully those familiar with a Roku and/or Playstation will have some ideas as to what can use data on those devices. Well, besides the streaming, of course.
When my son had a playstation, it would do updates that used massive amounts of our data. He had to turn off a setting that allowed the updates to start automatically.