As surprisingly enough, my group's Sanctioned Psyker didn't end up rolling a brand. However he does have a clan tattoo from his days as a Spyre hunter. So I just decided to let him use that, particularly as his history was one of comming from a Noble Family on Necromunda. I chose to let him not have been found to be a psyker until well into his late 20s-early 30s.

From Eisenhorn Omnibus - Malleus

'Look at the brand! The mark there!' I yelled, pointing to the Malleus rune burned into the child's ankle. The hammer of witches. The

brand-mark of the psyker.

Cobramax76 the witch you mentioned in the book isn't described, there are no references to a brand on her face.

the psykers captured and paraded during the tiumph in Malleus book are all chaos cultists, they were captured by a combined force of the inquisition led by an inquisitor from the Ordo Malleus.

The "hammer of witches" seems to be related to the Ordo Malleus, like some distinctive simbol used to simbolize their role in demonhunting, but branding someone with the hammer of witches is like saying: "ehi, he is a witch hunter! he wields the weapon that destroy the heretics!" or "through this mark i condamn you". If we see in christianity (that is school for inquisition) the cross was used to mark witches to make them suffer with the symbol of christ for their heresis, in the same way the cross was an holy simbol and was carried by those serving the church. I do not understand why the ankle, bad place to brand a person, the forehead or the clavicula of even the hands are better places and carry a deep simbolism too (forehead: impure thoughts to be purged from an impure mind; clavicula: the key of the soul or heart, to purge a corrupted soul; hands: to sanction impure actions and misdeeds). In my game those branded by any order of the inquisition recive a brand with the malleus inscribed in a circle, forehead, clavicula, hands are the places where the sibol is branded by the whim of the inquisitor passing judgment.

for sanctioned psyker i resolve with a tatoo placed on the face/head/neck, the tatoo is always luminescent and hiding it is a crime sentenced to death. In a universe paranoid about psykers you want to know if someone is and you want that everyone around knows that too, knowing that you are speaking with a person that is capable of reading your mind means you can decide to avoid or not talk, and if something goes wrong you can easly point to the psyker. then a highly visible brand makes sure that a rogue psyker has a difficult time going around without being indentified as soon as something around him is weird.