I have done some dark testing on my new ZWO 1600 Pro Mono camera. The attached dark is 300sec. Gain 300 Offset 50. The camera is not attached to the telescope it is covered by cap, two different caps have been tested, it is located in a dark room, and aluminium foil at the front to be sure that no stray lights reached the sensor. At the lower left part of the dark there is a bright sport with a surrounding halo, the spot is located at the same place for different caps. . The bright spot increases in intensity with lower temperature, and of course exposure length. The same pattern is visible at "high" temperature short exposures but with lower intensity.
I am 100% sure that no stray lights reached the sensor. For me it looks that there is either a large number of hot pixels or another defect on the sensor that create the light and that the halo is the reflection from that light being reflected by the sensor glass.
Any that can help me confirm or come with another explanation?
I have not tested this in a full imaging sequence so because I assume that the "defect" be removed by calibration, it will possible add and "artifact".
Her is the full frame autostretced in Pixinsight :
The bright spot: