I guess, I figured it out. Still not perfect but I know what's the prob.
Its the "zoom lens". The mechanics inside of the lens from the rear is not light sealed. So light may reach the sensor bouncing between the wall and zoom tubes. there are also small gaps at the mount end.
covering theat area and leaving a hole for the lens only - it helps a lot. I guess I'll need some additional cover for eliminating the prob completely. something like a mini lens hood...