In Rivendell we know a secret council between the different races took place regarding the ring. One thing that stands out to me is that in the book it implies that Sauron or at least his servants know that members of different races were heading to Rivendell.
"When you came north, Boromir, you were in the enemy's eyes only one stray wanderer from the south and a matter of small concern to him: his mind was busy with the pursuit of the ring" (the fellowship of the ring- a journey in the dark)
Now we know that Boromir set off from Gondor much earlier than when the ring actually arrived in Rivendell, so like Gandalf says he may just come across as a wanderer, but shouldn't the fact that Boromir or any other messenger of the different races heading to Rivendell be alarming for Sauron?! Down to fact all the different races will learn of his ring.