Before leaving Caladan:
"I'm going to watch our screens and try to see a Guildsman."
"You won't. Not even their agents ever see a Guildsman. The Guild's as jealous of its privacy as it is of its monopoly. Don't do anything to endanger our shipping privileges, <PERSON>."
Near the end of the first novel, when <PERSON>'s forces attack Arrakeen:
The taller of the two, though, held a hand to his left eye. As the Emperor watched, someone jostled the Guildsman's arm, the hand moved, and the eye was revealed. The man had lost one of his masking contact lenses, and the eye stared out a total blue so dark as to be almost black.
Guildsmen avoid the public and if they have to show themselves, then hide the usage of spice. In other books it is said that Navigators cease all contact with their families.
Later, when <PERSON> confronts the Emperor:
The Guild is like a village beside a river. They need the water, but can only dip out what they require. They cannot dam the river and control it, because that focuses attention on what they take, it brings down eventual destruction.
These passages imply that the Guild is keeping its total dependance on spice a secret. They took huge spice bribes from the Fremen to keep the skies clear of satellites. But even if they had to buy large quantities of spice openly, nobody could know if it went somewhere to be sold or mysteriously disappeared, because the Guild is the only one doing space travel.
Another exchange when <PERSON> finds <PERSON>:
"<PERSON>," he said, "are there many Guildsmen around Rabban?"
<PERSON> straightened, eyes narrowed. 381
"Your question makes no . . . "
"Are there?" <PERSON> barked.
"Arrakis is crawling with Guild agents. They're buying spice as though it were the most precious thing in the universe. Why else do you think we ventured this far into . . . "
"It is the most precious thing in the universe," <PERSON> said. "To them."
It seems to me like <PERSON> doesn't realize the spice's value to the Guild. It is something <PERSON> saw.
The Emperor didn't really know there is more to Arrakis than profits. 417