Everything is relative to perception, and your perception is limited.
The Ginger Ninja
Looking through the cards, I unfortunately cannot find any card that "unlocks" the Thick Fog card. Please name the card if you have encountered it.
Otherwise I assume it just gets taken off the map as soon as encountered because the card did not mention anything that it needs to be unlocked.
Jeez, ye of little faith
Thanks Rom. Could picture the card, but couldn't for the life of me remember its title.