Imagine being the Cohort soldiers from the Erebos who were sent respond to Judith's distress call.
They land to find a dead Lyctor, run through with a Cohort infantry sword, and two new Lyctors, one freshly missing an arm. I doubt either of them were particularly coherent by that stage.
And then they go to clear the inside of the building. In the room the transmission came from, there's a dead priest and an enormous pool of blood, but no sign of captain Deuteros. Her cavalier is missing an eye and seems to have been blown open from the inside.
A room down the hall is singed and splattered with blood and chunks of human flesh. Perhaps there are fragments of grey robes, or perhaps some poor psychometrist works out that they're looking at what's left of the Master Warden of his House.
Further into the building they enter a study with the words "YOU LIED TO US" daubed across an ancient and beautiful mural. The Third House cavalier lies dead on the floor, stabbed from behind. The Master Templar of the Eighth is lying dead, his throat slit, apparently by his own cavalier's sword. And his cavalier... His eyes are gone, there is something wrong with his mouth. His wrist and neck are broken. The whole room is dripping and sticky with blood and human fat.
Searching past the kitchen, they find the morgue. There's a bowl of ashes (two people's, dead before the pilgrimage even began, confirms the by now very shaken psychometrist). One of drawers lies open and the sheet has been roughly pulled off the body inside: the utterly shattered body of the Fifth House necromancer is lying there, her blouse rolled up to her ribs, a fist sized hole in her abdomen.
Neatly lying under sheets in the other drawers there are more bodies, and the preserved severed head of the Seventh House cavalier. There is no sign of his body. The Fourth House cavalier has been impaled through the chest, shoulders and legs, precisely, like an insect for display. Her necromancer...it might be easier to list the places where he hasn't been impaled. The Fifth House cavalier is just as destroyed as his necromancer: limbs broken, body horribly mangled.
Later, they find the bloodsoaked bed with "sweet dreams" daubed on the wall in blood. If they get as far as the facility, they discover the outlines of two horribly broken bodies surrounded by necromantic diagrams drawn on the floor in pen. One unremarkable room is splattered in blood and singed with spirit fire.
The building is full of collapsed skeleton constructs, seemingly mid task, as if all struck down simultaneously, and as they explore they find more dead priests. They find no sign of the Sixth or Ninth cavaliers, or the Crown Princess of the Third, or of Captain Deuteros. And from what they've already seen, this can't feel encouraging.
It's clear that this building has witnessed necromantic horrors beyond their comprehension. What were the scions of the Houses doing, or what was being done to them? What could possibly cause what they have seen?
And I can't imagine that after seeing the truth of what happened at Canaan House, that John would have taken the risk of those soldiers revealing what they had seen. After all, he's a very careful guy.