[narfforc]

When The Witch-king led his troops out of Minas Morgul, he stopped for a moment sensing another power in his vale, this was of course missed in the film due to moving him from horse to winged beast. He was more sensitive to the presence of the ring than the other nazgul, and at this point a whole lot closer. Sauron's mind was elsewhere, directing all his minions and thinking about which of the wise had found the ring, he knew of Gandalf, Denethor and had only just been notified of the existance of Isildur's Heir, he was not above doubt. He could not think for one minute that anyone would destroy such a powerful weapon, what would anyone be doing in Mordor, he believed it was impregnable. If one of the wise had put the ring on in Mordor I think Sauron would have sensed that, but the ring only gave power by stature (not size), Frodo was no ordinary hobbit, he was wise and learned, the ring held more danger for him than Sam. I think that so little energy was given off by the ring when Sam wore it outside or inside Mordor, that it would probably go unnoticed unless in the direct gaze of The Eye.