I agree with your statement regarding planks. Not enough people realize the benefits to bridge work and other isometric core holds. Not only do they strengthen the abdominal wall, but they work local core muscles as well that are often underdeveloped.
However, you do gain fat beneath the muscle. This is visceral body fat, fat that is held around the organs in the abdominal cavity. Its most appearent in the "beer belly" condition, especially in men, where the stomach is hard, but very large and bloated. This fat is the most pathological, and increases the risk of nearly every cardiovascular and metabolic disease.
To the original poster, when you contract your abdominals, do you still see protruding in the lower abdomen? If so, this may be a sign of visceral fat storage. Most people, however, have a slight angle of protrusion from the upper abdomen to lower when standing relaxed, this is due to the intestines and organs, especially if the you just ate a meal or if there material in the intestines.
Br