I just wanted to add this.
 
I set at idle doing nothing I set at 1% CPU, 1% GPU
If I move the mouse the windows manger uses like 16% GPU
 

Again, I don't think that is abnormal for your low resolution and "Optimal Power" setting.

Still seems weird that the GTX780 and this same monitor stayed at 0%

Changing the 3D setting for "Power Management Mode" has no effect on it...
 

Still doing this with 2 new ASUS VG278Q monitors.
Most everyone I talk to has the same thing going on, so I guess it's normal.

I did a secure erase on my SSD, and Installed Windows 10 1809 latest.
Installed Chipset Drivers, and Nvidia Drivers.
Still exists...
 
BTW I did not reinstall windows due to this, it was just time to do it.

Did you ever solve this?

I am seeing a very similar situation with a 1080 card (while I await notification on a 3080).
 
A couple of times a week my computer freezes.  Before it it totally gone, I can see the task manager showing 100% GPU.  There are no processes showing any usage when this happens.

Same thing on my 2070S I thought it was discord or firefox so I disabled hardware acceleration but not working.

Anyone with this issue running SLI?  Some applications don't know how to handle the configuration.  i.e. DX12.  There might be times you sit at 100% GPU in games simply because in optimization, SLI was completely overlooked :points at self: me being one of them.  My only work around is to run DX 11 legacy in those games.

To start troubleshooting this one, I'd go into safe mode and see how the GPU is acting without hooks into the operating system.