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It might not be intended for the public.

Similar to your printer fingerprinting: https://en.wikipedia.org/wiki/Printer_tracking_dots

You can reveal it later when you come up with a new mechanism and out all the fake images. Basically the first layer is first defence, second layer is cleanup when releasing a new mechanism. That way your generated images will always be identifiable eventually.

It's to mark the images for exclusion during training.