CCD's operate using the property of silicon to convert light to electrons, but this also operates in reverse, silicon circuits can emit light when operating.
At the corner of the CCD array is a high gain amplifier that converts electrons to a voltage that can be measured by the A/D converter. During the exposure this amplifier can emit enough light that it can be seen as a glow in the corner of the frame in long exposures.
To check for amplifier glow simply take a long dark and stretch the image to look for any bright areas along the periphery of the array. The glow is typically in the upper left, but light sources can appear anywhere along the periphery depending on the location of the amplifier and any other structures of the CCD. Both images at the left are linked to actual dark frames showing this effect.
The usual solution to this glow is to power down this amplifier during the exposure, this feature must be designed into the camera and is usually not under user control.