Sounds like it's a dust mote or two. I get them fairly regularly on my DMK as well, especially when changing it between barlows or using it as a guide camera then back to an imaging camera etc.
To clean, I use a q-tip dipped in demineralised water, wiping it across the face of the CCD from one side tot he other in strokes.
I then sit it vertical so the water slides/dries off the side. This usually gets rid of all the dust for me.
Sometimes the water doesn't completely slide off and it ends up worse than when I started, so I just do it again.
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