What was the particular risk in the road out of the Isenmouthe?

Question

Some days after they venture into Mordor, Frodo and Sam run into an Orc army and are forced into its ranks. They manage to get away quite near to the Isenmouthe, and then the head east beside the road that leads from the Isenmouthe to Barad-dûr. After a while, though, they tire:

The hobbits had gone a few weary miles when they halted. Frodo seemed nearly spent. Sam saw that he could not go much further in this fashion, crawling, stooping, now picking a doubtful way very slowly, now hurrying at a stumbling run.

"I'm going back on to the road while the light lasts, Mr. Frodo," he said. "Trust to luck again! It nearly failed us last time, but it didn't quite. A steady pace for a few more miles, and then a rest."

He was taking a far greater risk than he knew; but Frodo was too much occupied with his burden and with the struggle in his mind to debate, and almost too hopeless to care.

_{The Lord of the Rings Book 6 Chapter 3: "Mount Doom"}

I've emphasized the part I'm confused by. I imagine the risk that Sam "knew" was that they could run into another one of the armies of Sauron, or that they could be spotted and attacked by one of the Nazgûl, so the "far greater risk" must be something even worse. However, on the road they ironically aren't met with any danger at all so there's no indication, to my memory, of what this particularly great risk was.

Why was Sam "taking a far greater risk than he knew" in choosing to walk the road out of the Isenmouthe?

Answer 1

This is appropriately answered a couple of sentences on:

But their luck held, and for the rest of that day they met no living or moving thing; and when night fell they vanished into the darkness of Mordor.

(Lord of the Rings, Book VI, Chapter 3, "Mount Doom"; emphasis added)

It appears, thus, that the risk was of meeting something on the road. Indeed, we're told in the previous chapter that

Along all the roads troops were moving; for the Captains of the West were advancing and the Dark Lord was speeding his forces north.

(Book VI, Chapter 2, "The Land of Shadow")

It seems that the narrator believes (or represents) that there was a significant chance of Frodo and Sam running into another of these orc-companies while they were on the road, and being unable to escape as they had previously.

Answer 2

The sentence after the one you highlighted seems to state that Frodo did know about the greater risk but lacked the initiative to bring it up.

The only risk I can think of that Frodo could know and Sam could not is discovery by Sauron himself through magical means (such as the Minas Ithil Palantír). Frodo has felt the direct attention of the Eye of Sauron before when the wore the Ring at Amon Hen, and might suspect that this much closer to Barad-dûr, Sauron might be able to discover them on open ground even without help from the Ring.

Backing this theory is Gandalf and Aragorn's decision to mount a foolish-seeming attack on Mordor itself for the sole purpose of diverting Sauron's attention towards their army:

But we must at all costs keep his Eye from his true peril

_{Gandalf in Book 5 Chapter 9: "The Last Debate"}

Answer 3

I think the big risk is simply that there are lots of Orc-companies on the road, and if they find any hobbits, Sauron gets the Ring and Middle-earth is lost forever.