Inquisitor Marr told you that the Inquisition would have other agents besides you in the House of Dust and Ash:

The most visible of which is Inquisitor Herrod and his retinue. The Inquisitor, wearing a hooded white-and-gold robe and full regalia can be found wandering through the halls and corridors of the House at all hours. His guards and other staff are attending him, as well as patroling the grounds and - on occasion - speaking to people or examining objects up for auction (that part is probably mostly for show, since the Inquisition has surely examined the collection already).