Aragorn's Dilemma

[Maédhros]

And if Aragorn had gone with Frodo and Sam I'm sure the whole thing would have failed. I don't see how Sauron couldn't notice Isildur's heir walking into Mordor.

If I seem to remember correctly:

Now the flicker of approaching torches and the clink of steel ahead was very near. In a minute they would reach the top and be on him. He had taken too long in making up his mind, and now it was no good. How could he escape, or save himself, or save the Ring? The Ring. He was not aware of any thought or decision. He simply found himself drawing out the chain and taking the Ring in his hand. The head of the orc-company appeared in the Cleft right before him. Then he put it on.

So, if Sauron couldn't notice that Sam used the ring, why would he be able to locate the Heir of Isildur.