When Frodo and Sam fall in with a band of Orcs between the Castle of Durthang and the Isenmouthe (before the battle of the black gate), they escape after creating confusion between the Orcs and the Uruk-hais. They were made to join the group of Orcs as they were thought to be stragglers and they were dressed in Orc clothing. Why couldn't the Orcs/Uruk-hais catch their scent even when the hobbits were so close? (As far as I know, they had a pretty strong sense of smell and could smell human flesh from miles)
This is mainly conjecture, but:
- Frodo and Sam probably hadn't washed since Ithilien.
- They had been through Shelob's Lair.
- Frodo had been through captivity in the Tower of Cirith Ungol.
- They were both dressed in filthy Orc rags.
Additionally, Tolkien notes in this chapter that:
The stench of the sweating orcs about him was stifling.
In other words, Frodo and Sam would have smelled bad enough to begin with, and the surrounding Orc-stink did the rest of the job.
Orcs were also quite busy and did not pay attention, the great part of those marching in this specific unit were supervised by Uruk drivers with whips who hurried to get the march done (and those with whom Frodo and Sam were walking were those unwillingly serving Sauron in his wars, caring only to end the forced march and escape the whip, those orcs ,,native" to Mordor would be more disciplinarian, indoctrinated in Dark Lord's service enough to discipline other orcs at the mere sign of ,,rebel talk"), their orders were clear they were suppose to march to Udun at all speed, they couldn't care less if any among their ranks were acting, looking or smelling differently (hobbits were smaller even than smallest orcs, but as the orc had a habit of running with their back bend, they appear smaller than they are, in The Hobbit we are often told that mountain orcs/Northerners/Goblins are big, larger than dwarves and Bilbo, also Frodo and Sam took clothes of the smallest possible orcs and yet those clothes were still quite oversized for them but servicable enough to pose as them) though with orc clothing and the general stench it would be more difficult for them to notice (but indeed orcs can be ,,as keen as hounds" when tracking on scent, and there are even specialized orc trackers, trained for the purpose).
Quick now. Mr. Frodo!’ he said.I’ve had a bit of a search to find anything small enough for the likes of us. We’ll have to make do. But we must hurry."
,,He opened the bundle. Frodo looked in disgust at the contents, but there was nothing for it: he had to put the things on, or go naked. There were long hairy breeches of some unclean beast-fell, and a tunic of dirty leather. He drew them on. Over the tunic went a coat of stout ring-mail, short for a full-sized orc, too long for Frodo and heavy. About it he clasped a belt, at which there hung a short sheath holding a broad-bladed stabbing-sword. Sam had brought several orc-helmets. One of them fitted Frodo well enough, a black cap with iron rim, and iron hoops covered with leather upon which the evil Eye was painted in red above the beaklike nose-guard."
The Morgul-stuff, Gorbag’s gear, was a better fit and better made,’ said Sam;but it wouldn’t do, I guess, to go carrying his tokens into Mordor, not after this business here. Well, there you are, Mr. Frodo. A perfect little orc, if I may make so bold-at least you would be, if we could cover your face with a mask, give you longer arms, and make you bow-legged. This will hide some of the tell-tales.’ He put a large black cloak round Frodo’s shoulders. `Now you’re ready! You can pick up a shield as we go.’"