In chapter two of The Lord of the Flies, the boys decide that they need to get organized to build a fire on top of the mountain to act as a signal. They hope that the smoke from the fire might be seen by a passing ship or plane and then lead to them being rescued.
As they start to build the fire, they get carried away and build a huge fire that quickly exhausts their store of ready fuel so they run off into the woods to find more wood for the fire. This disorganization is what likely leads to the loss of the kid with the mulberry colored birth mark.
It is when Piggy notices and points out the fire that spread down into the jungle that the boys also notice that the boy with the birthmark is missing. There is no physical evidence of his death, but only the fact that he is absent and no one knows his name. The huge fire stands out as the likely culprit in his death.