The location of the Demon Slayer Corps Headquarters was referred to as the estate of the Ubuyashiki family by Rengoku Kyoujurou.
Because the leaders of the Demon Slayer Corps are descendants of the Ubuyashiki family, who have held the position for generations, and since the corps itself was originally formed by this family, the place where the leader resides also serves as the headquarters—the Ubuyashiki family estate.
Given that the exact location of the estate was not to be easily disclosed, Reiger was not informed of its whereabouts on his way to the Demon Slayer Corps Headquarters.
What's more, Reiger didn't even have the opportunity to see the route to the Ubuyashiki estate; he sat with Rengoku Kyoujurou in a carriage that was tightly covered and was transported all the way to the destination.
Thanks to this, when Reiger stepped out of the carriage, he had no idea where he was or what place this was.