It seems that if a familiar can hide then it can stay hidden no matter what terrain it passes through as long as you don't use the visible command to have it step out or attack anything. I had my familiar hide and it walked from Galadon to the Forest of Nowhere in the past without ever stepping out of the shadows. Even though thieves and assassins would step out when they go from civilized/plains to wilderness/water terrain. My familiar didn't even step out when it passed through the pond rooms in the Holy Grove. I'm thinking this is a bug. As much fun as it would be to abuse it.

n/t