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Probably just some crud.Perhaps your shutter is shedding a small amount of material?
Your filters are pretty dirty by the way. Some filter wheels have cutouts in their filter wheel. I have read where this can cause the dust donuts to move around from the wind of the filter wheel moving. This would then not be able to be flat fielded out which assumes the donuts don't move. I find my FLI filter wheels are very good for this. Clean the filters well, install them and then 3 months later they are still clean. Its why I got a 7 position filter wheel even though it was expensive as it stops the constant opening and closing to fit Narrowband then take them off again.
I use a Hurricane blower and a photographic micro fibre cloth to clean them and inspect them with a bright torch illuminating them at an angle to get a reflection to show up spots. I am for a fully clean image without the need for dust donuts to be removed with flats. It takes a lot of pressure off of the calibration process.
Whilst flats may remove dust donuts on some systems unless your darks and biases are perfect and your flats taken really precisely they won't. Also you've got to wonder if the crud is large enough to make a black spot that large does the signal ever get fully restored if it never fully arrived in the first place?
Greg.
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