Need help, weird spot on recently bought monitor

Dear members of the forum,

I recently bought a 2nd hand monitor.
Now I know you need to look out for defects on the monitor before you buy it.
Unfortunately I was in a hurry and hooked my laptop up to it and all was fine with the picture.

Last night is set up the monitor to my main rig and discovered something.
When the monitor is on but not receiving video there is a weird spot on the monitor.
It about 2 cm in length and 0.5 cm in height.

The spot only shows when there is no video sent to the monitor.
Does anybody know what this is????