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I normally use ISO 400, 800 and 1600 and have a number of darks for these at various times. For the matches at 10 and 30 seconds, my images look very similar to yours. Most of the image temps range from 22 to 27 degrees.
I just did a test at ISO 6400 at 30 sec... (temp here is about 27) and it was like yours - no noise (as in I can not see ANY noise on the black image). It's much clearer than any other dark I've done at the lower ISO. I do not have long exposure noise reduction on, but looking the properties of the image it does have the high ISO noise reduction on standard. Haven't tried the extra test as you did - I was just surprised by that result.
...time passes as I test things...
I did some more darks for 30 sec - 3200, 1600, 800 - all have noise like my other darks. I did one for 25600 and it has far less noise than the lower ISO images.
I'll have to try an ISO6400 night shot sometime to see how it looks.
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