Thanks all. Just to reiterate that the photo in my first post shows a whole in the outer (ie, room-side) plasterboard which reveals the polystyrene insulation and the mystery material behind, which when tapped/knocked feels very solid (ie. it’s not possible to cut a section out).
In this pic you can see where the interior plasterboard and insulation ends by the door (the door is open - the outside of the building is on right, the inside on the left). The end of the knife is touching the mystery material which is uniform all the way down. It’s flush with the exterior wall (white on the left).