Task Manager shows higher total CPU usage than the sum of all running processes. Hidden process?

Solution 1:

This is because the task manager round up or round down the CPU usage values per process. You can see a lot of process using "0" CPU time, this is false, they use "0,xxxxxxx" cpu time.

What you need is a process manager showing more precise value to you.

Some process can also be hidden (negative PID, running as another user) or some piece of hardware may have direct access to the CPU by passing your operating system (but probably not)

Solution 2:

Run Process Explorer and you'll see the cpu usage for everything including services like anti-virus checkers. (I just had the same issue and I found Symantec's ccSvcHst.exe was taking up 13% of my cpu.)