It was most likely hemangiosarcoma. This is a tumor of blood vessels, most commonly seen in the spleen, then the heart, skin and brain.
When the tumor grows, it has a tendency to bleed. When it is found in the heart, it bleeds in the sac around the heart. The blood fills up the space the heart needs to move, and the heart can't fill up with blood, and the body doesn't get enough oxygen.
If I were to pick a way to go of natural causes, this would probably be close to the top of my list. Its common to see dogs pass away quickly and peacefully from bloodloss due to hemangiosarcoma. Its also very common to see ZERO symptoms prior to finding massive bleeding, so you probably didn't miss anything.
It doesn't make the loss of your pet any easier, but take comfort knowing you pet probably didn't feel much/any pain, he was just tired and went to sleep.