I agree with you, I think the person either drove (walking is possible but unlikely) on to the island and quickly disposed of the body on the beach. What is baffling to me is that there seem to be so many other places that would be a better hiding spot for the body...why there? Look at how out in the open the body is:
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At first I thought maybe someone had to place her somewhere temporarily for some reason and planned to come back and get her to finally dispose of her somewhere else but something got in the way of that plan and she was found before they could get back to there. I thought maybe a night shift worker at the plant on the island, who planned to get the body from the beach and dispose of it elsewhere early in the morning once his shift was over. But looking at where the body was left, that makes no sense as it is too out in the open. Surely they would hide the body somewhere more concealed? This is so puzzling. IMO.