They're covered quite a lot in Out Of The Darkness. Cerebral Implants. Pertinent quote:Really? I was only aware of the Guardians having brain implants. Possibly the caretakers were originally attracted to Halsey by the fsd drive, which seems might well have been sabotaged & possibly giving out out hyperspace signals; it exploded during the witchspace journey (reported on galnet by one of her accompanying escort flight). If they are the disembodied guardian AI then it makes sense for them to contact/control a spaceship technician; they probably want to get some form of body/ship.
"Implants were supposed to be secure, inaccessible from the outside"
(Just for clarity, the wording which implies that they might not be secure is due to a character worrying about whether theirs is truly secure, not due to them not actually being secure.)