How come Rivendell was never discovered by Sauron?

[Elladan and Elrohir]

"Yet things might have gone far otherwise and far worse. When you think of the great Battle of the Pelennor, do not forget the battles in Dale and the valour of Durin's folk. Think of what might have been. Dragon-fire and savage swords in Eriador, night in Rivendell. There might be no Queen in Gondor. We might now hope to return from the victory here only to ruin and ash. But that has been averted -- because I met Thorin Oakenshield one evening on the edge of spring in Bree. A chance meeting, as we say in Middle-earth." - Gandalf, to Frodo and Gimli; The Lord of the Rings, Appendix A (italics added)