No-one notified them. It was a "mentat projection" (viz. a guess) by Hawat, based on the following facts;
- The Harkonnen wouldn't have given up the planet if they didn't have a plan to take it back
“The Harkonnens took ten billion solaris out of here every three hundred and thirty Standard days.”
A muted gasp ran around the table. Even the younger aides, who had been betraying some boredom, sat up straighter and exchanged wide-eyed looks.
Halleck murmured: “‘For they shall suck of the abundance of the seas and of the treasure hid in the sand.’ ”
- The Emperor personally ordered them to take over Dune, suggesting his direct involvement in a plot against the Atreides Duke.
“You see, gentlemen,” Leto said. “Is there anyone here so naive he believes the Harkonnens have quietly packed up and walked away from all this merely because the Emperor ordered it?”
- The Harkonnen don't have sufficient troops to attack the Atreides directly. Their forces have been dramatically depleted by Fremen attacks and despite their numbers, they weren't much good to begin with.
“There’s nothing definite,” he said. “They wish to observe us for a
while, Duncan believes. They did, however, promise to stop raiding our
outlying villages during a truce period. That’s a more important gain
than it might seem. Hawat tells me the Fremen were a deep thorn in the
Harkonnen side, that the extent of their ravages was a carefully
guarded secret. It wouldn’t have helped for the Emperor to learn the
ineffectiveness of the Harkonnen military.”
- The Landsraat don't support the Emperor or the Baron, leaving the Sardaukar as the only forces at the Emperor's disposal.
“Our enemies have a head start—too much of a lead to overcome.”
“The Emperor,” Paul said. “That means the Sardaukar.”
“Disguised in Harkonnen livery, no doubt,” the Duke said. “But the soldier fanatics nonetheless.”