I am in the middle of the 3rd Dune book, Children of Dune. I do not understand how Lady Jessica/Gurney knew that Leto II would go to the desert, the time and location. I may guess that the Lady Jessica deduced it after her discussion with Ghanima, and communicated this to Gurney, but maybe it was already known to her somehow? Also, she sent Gurney to the Sietch before that, didn't she?
There is a trail of clues.
At the return of Jessica to Arrakis, Jessica and Stilgar enact a plan to judge the loyalties among the crowd attending the event. Some are arrested for betraying themselves by details of their behaviour.
Some turned out to be dullards, some the usual suspects, others Alia's people. More interestingly, some reacted to interrogation in a very suggestive way, Gurney relates:
There is another thing, quite disturbing. Under the deep drugs, some of our captives talked of Jacurutu and, as they spoke the name, they died.
The rumours of Jacurutu had previously just been dismissed as folk-legend, but this was too compelling to ignore.
Jessica to Gurney:
Resume contact with your friends among the smugglers. If Jacurutu exists, they'll support themselves by selling spice. There'd be no other market for them except the smugglers.
I've already set this course in motion, My Lady.
Later, when the Lady Jessica is first able to speak to her granddaughter Ghanima alone, they speak of Leto, Alia - and on one occasion Leto's reaction to Alia:
"She works a strange attraction on Leto," Ghanima said. "That's why I helped you meet alone with me. He agrees that she is beyond hope, but still he finds ways to be with her and . . . study her. And . . . it's very disturbing. When I try to talk against this, he falls asleep. He --" "Is she drugging him?" "No-o-o." Ghanima shook her head. "But he has this odd empathy for her. And . . . in his sleep, he often mutters Jacurutu." "That again!"
Planting the thought in Jessica's mind of Leto's interest in the place of the legend of "The water Insects" - water stealers, the cast-out - Jacurutu.
Then, on the occasion of her private meeting with Leto:
"It'd be better for me never to become Emperor," Leto said. "Oh, I don't imply that I've made my father's mistake and peered into the future with a glass of spice. I say this thing out of selfishness. My sister and I desperately need a time of freedom when we can learn how to live with what we are."
The thought is planted in Jessica's mind that they seek a means of escape from their duties, their surroundings. Where else could they go (on-planet) except into the deep desert, and where else could they seek shelter where they would be beyond the reach of Alia's spies and the tribes loyal to the new Imperium but Jacurutu?
Then at Sietch Tabr, Leto to Stilgar:
"I'm an adult in a child's flesh," Leto said. He pointed down to a narrow crack in the rocks below them. "If I die here, it will be in that place. You will see the blood. You will know then. Take my sister and --"
No sense of when it might happen, and it's not certain if Stilgar passed this information on.
The "when" part (since Jessica and Gurney could see it coming) had to be taken from subsequent events which precipitated Leto and Ghani's flight to the desert from Sietch Tabr and Ghani's return without Leto.