Hi Peter,
DO NOT take shots with the lens cap on and expect dark clean images
.
Why i hear you ask
, its because IR light goes right thru the cap. Not to mention that the lens cap is not totally light proof. The cap serves only one function, to stop dust setting on it.
Even the tiniest light thru a pin head, will show up like what you have.
With your moisture issue, are you getting a dew spot on the outside of the filter, or on the CCD ?.
If its on the CCD, then theres a air leak. If its on the front of the nose/filter, then there are other ways to get rid of this for your humidity levels during the night.
If you need more help, send me a PM and we can take look at it properly.
Theo.