No other computers in the house besides the laptop? What is the laptop used for when it is on? Windows or Mac? No other smart devices that use data for anything?
If your TVs are smart TVs, maybe disconnect them from the network as well. It's not unheard of for smart TVs to use data without telling you. If the laptop isn't regularly used it may be less of a possible issue but I'd make sure it's got up to date antivirus software on it and the computer isn't using data. If it's Windows 10, you can see the network utilization in Task Manager under the Performance tab. If it's using a lot of data, there could be an issue on the laptop also.