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I just saw the movie Dune (2021), and I also read the first book a few years ago. I was surprised that one of the closest persons of the Atreides' family, Dr. Yueh, didn't get any surveillance, such that they were not aware his wife was captured by the enemy Harkonnen (moreover in a period of great tension).
Is there any explanation later on in the work or is it just incoherent?
In brief, Yueh is a person who is supposedly above any consideration of malice or duplicity toward his patients, the house Atreides, because of the conditioning that Doctors from the Suk School receive.
IMPERIAL CONDITIONING: a development of the Suk Medical Schools: the highest conditioning against taking human life.
Dune
There's no point investigating him as a potential traitor beyond confirming that he's the real Yueh.
“Hawat will have divined that we have an agent planted on him,” Piter said. “The obvious suspect is Dr. Yueh, who is indeed our agent. But Hawat has investigated and found that our doctor is a Suk School graduate with Imperial Conditioning — supposedly safe enough to minister even to the Emperor. Great store is set on Imperial Conditioning. It’s assumed that ultimate conditioning cannot be removed without killing the subject. However, as someone once observed, given the right lever you can move a planet. We found the lever that moved the doctor.”
Dune
Even when it becomes blindingly obvious to anyone with eyes that Yueh must be the traitor, Paul still can't understand what's happening. He thinks he's mistaken, or that it's some manner of ruse.
He wondered who could’ve captured her. His own captivity was plain enough– to bed with a capsule prescribed by Yueh, awaking to find himself bound to this litter. Perhaps a similar thing had befallen her. Logic said the traitor was Yueh, but he held final decision in abeyance. There was no understanding it–a Suk doctor a traitor.
The answer by Valorum explains clearly why Hawat didn't put Yueh under stricter surveillance; the part of the question regarding Yueh's wife is addressed during a conversation between Hawat and Lady Jessica:
"You know that's an empty gesture," Hawat said. "He's conditioned by the High College. That I know for certain."
"Not to mention that his wife was a Bene Gesserit slain by the Harkonnens," Jessica said.
"So that's what happened to her," Hawat said
This makes it clear that the mentat did investigate Yueh's past - he simply wasn't able to find information about Wanna's fate.
It's not incoherent. It's very clearly explained.
Yueh had Imperial Conditioning, which is supposed to be unbreakable. Even if they had known about his wife - and he does in fact reveal it to Jessica at one point - they still would not have considered him a potential traitor.
They didn't suspect Yueh because he had had Imperial Conditioning to not betray the Atreides. However, Piter De Vries had manipulated him into not viewing his actions as a betrayal; Yueh had come to the conclusion that the Harkonnen were so powerful and determined that they were going to succeed one way or another, and the interests of the Atreides would be best served by appearing to go along with the Harkonnen plan to get an opportunity to kill the Baron and save Jessica and Paul.
