Fat sinking to bottom of stock
Gelatin. All bones and connective have gelatin and collagen. When you cook bones slowly and gently for a long time, the gelatin leaches out in the water. Being heavier, it sinks. It isn't marrow. Marrow will stay in the beef bone until you spoon it out (lucky you). If it did fall out, it would stay in one piece. But I believe the substance you found in the bottom of your soup pot is the natural gelatin from the bones in your stock!