It's the in-camera Dark Frame Subtraction.
The camera is taking another photo in the dark, with the shutter closed.
It then subtracts this 'dark' photo of the sensor noise and hot pixels from the 'Light' photo.
http://en.wikipedia.org/wiki/Dark-frame_subtraction
Unfortunately, looks like it may not be something you can turn off in the K-x:
http://www.iceinspace.com.au/forum/s...ad.php?t=88187