On Weathertop: Sauron doesn't find Frodo, but Frodo puts the Ring on in the presence of the Nazgûl who are then able to physically harm him. On Amon Hen: Sauron doesn't find Frodo; Frodo's gaze is drawn towards Barad-dûr, Frodo feels the Eye, and only at that point does Sauron become aware of Frodo. More @Wikipedia
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