Obscure, obscure, obscure.....
Following the Downfall of Numenor, the Exiles in Middle Earth realised that Sauron had also escaped by seeing the smoke rising from Mount Doom? (Clumsily worded, but you can hopefully guess what I mean.
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Or, simply, Sauron fled there to rebuild his power? (Though I s'pose that's all of Mordor.....hmmm.....)