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With operation at extremely low signal levels and the complex solid state physical phenomena that have variously been seen in CCDs, it is hard to fully model all other instrumental effects that might be present, either at beginning of life or after degradation in orbit.
Bias frame patterns can be seen whereby the absolute signal (in ADC units) measured for nominally empty pixels can change across the array. This can be due to effects such as incompletely subtracted a.c. time constants of the off-chip circuits, interaction of clock feedthroughs with the video signals, etc. Clearly any temporal changes due to thermal drift component degradation might be expected to alter these values with time.
Infrequently there may be light emitting defects which create a local background signal (usually these emanate from the amplifiers or the chip edges). Another local background signal enhancement may be spurious charge generated by an impact ionization process in high fields generated by the clocks.
These and other artifacts may be examined by regular monitoring of calibration frames telemetered to the ground in raw or histogram binned forms.