Good question. Maybe it was just luck, or maybe Isildir could 'sense' it. If Isildur had sensed Sauron, maybe he knew the general area and got somewhat lucky. Or here's another possiblility; maybe it was destiny. I really don't believe in destiny, but it was Tolkien's work and maybe he did, or maybe Tolkien just wanted to put it like that.
As for the movies, I think PJ should not have showed Sauron. Since this is not the movies section, I really won't get into that (and I'm too tired anyways).
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Do Not Touch
-Willie
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