There are things that a laptop or computer can do even when you are not doing anything yourself.
Lots of software checks the internet on a regular basis to see if there are newer versions of itself available. Java, Adobe Reader etc all do this.
Your security software could be checking the internet for updates, or indeed doing a security scan.
Windows could be indexing files, to make it quicker and easier to find files if you do a search.
Windows could be running a "Windows Update" to see what files need installing, and downloading them and installing them.
All these things can be going on, even if you are not touching the machine.
One way to check what is running is to press Ctrl + Alt + Delete all at the same time and select Task Manager.
On the Task Manager screen select the Processes tab.
This shows you what Processes (parts of programs) are running. You should select "Show processes from all users" at the bottom to see all processes.
Click the "title" of the CPU column to put the highest numbers at the top.
The list will change on a second by second basis, but you will soon see what processes are using the most CPU.
Search the web to see what they are, or put the names here and we will tell you what they are.