Taking dark frames at home 1200sec @-10C I noticed white marks appearing randomly. Any idea what causes these? These pics are from a small area of the Darks
Colin is correct. That's why we recommend using median or sigma clip when stacking, it gets rid of them.
I see similar one on 600 seconds dark frame (I'm in the process of fully redoing my dark library after the latest firmware upgrade).
Colin, Doug do you suggest that I have cosmic or gamma ray inside my room?. Is there a way avoid them?
A few kilometres of rock above you will help a bit with the cosmic rays, and heavy lead (Pb) or paraffin shielding for the gamma rays. ;-) They are a natural occurrence, and can't really be avoided. Save a few of them to share with your friends - the cosmic ray photons came a long way to light up your camera's pixels.
Just be thankful for the earth's atmosphere. This does get worse at higher elevations. Some of the radiation may actually be from building materials like concrete and drywall. There's some level of natural radioactivity in most materials.