During Season 2, we learn that Susan Ivanova is a [weak] latent telepath. Given the highly systematic scanning seemingly carried out by the Corp and Earth Force's own strong regulations against telepaths in normal service, how did Ivanova manage to have a successful career as part of Earth Force?
Her mother taught her how to fool the detection tests and she was moved from school to school to help avoid detection. This was mentioned in the episode, Divided Loyalties
But it goes beyond just that. That gets her to enlistment.
It's noted in several episodes that use of Telepathic scans is limited to criminal trials and command level personnel, and only to answer to specific charges.
You need to do SOMETHING to get noticed.
Ivanova further avoids use of her own abilities - she avoids any effort to use it. She avoids association with telepaths, at least until her season 2 budding friendship with Talia Winters.
Further, it's implied she's got a rep for being by the book; you can't legally be scanned if you don't break the rules.
Further, when she tells Bester to get out of her head, he doesn't react as if she's a psi; he seems only mildly surprised, until he recalls she's the daughter of a telepath. He could have, had he believed she was a true talent, taken her in right then and there, or sent word to have her transferred where he could safely detain her.
We also hear repeated mentions of the ability to resist scans by normals - and notes that it's not going to resist even a P5 making a dedicated attempt. But this implies strongly that casual scans can be blocked, and unless there is soe reason to look for it, a normal not being read isn't something most will look for.
Some of the subsidiary materials, like the Role-playing games, note that Psi Corps has developed genetic tests which find the most common markers, and these are used in the schools as wide net screenings. It's noted also they have a large false negative rate (30%), but few false positive results. It's possible that either she wasn't screened, or was a false negative. The other screening methods require active participation or active scanning by a telepath.
As for the genetic tests, it takes a while for someone in a bureaucracy to notice that some piece of paperwork is missing, possibly years. As long as they move before the bureaucrats notice the missing genetic test they could avoid notice for years.
By then Susan probably was well trained enough by her mother to fool the test and hide her talent. So even though she would by that point be flagged on the genetic tests once her mother was noticed, subsequent tests would fail to notice her talent.