The real Lord Renoux didn't originally live in the mansion. Instead he lived in the Farmost Dominance. Once replaced by OreSeur, he "bought" the mansion and moved to Fellise. Kelsier explains part of this in Chapter 6. He tells the crew that he killed Renoux and that he is now being played by an imposter, but fails to mention explicitly that it's a kandra.
“His name is Lord Renoux,” Kelsier said. “He arrived in the area a few days back. He’s staying in Fellise—he doesn’t quite have enough influence to establish himself in Luthadel. Besides, I think it’s prudent to keep Renoux’s activities a bit removed from the Lord Ruler.”
Vin cocked her head. Fellise was a small, suburb-style city an hour outside of Luthadel; she and Reen had worked there before moving into the capital city. How had Kelsier recruited this Lord Renoux? Had he bribed the man, or was it some sort of scam?
“I know of Renoux,” Breeze said slowly. “He’s a Western lord; he has a great deal of power in the Farmost Dominance.”
Kelsier nodded. “Lord Renoux recently decided to try and elevate his family to high noble status. His official story is that he came south in order to expand his mercantile efforts. He hopes that by shipping fine Southern weaponry to the North, he can earn enough money—and make enough connections—to build himself a keep in Luthadel by the end of the decade.”
A few sentences later Breeze realizes the real Renoux is dead
Suddenly, Breeze laughed. He leaned back in his chair, downing the last of his wine. “You blessed madman! You killed him, didn’t you?Renoux—you killed him and replaced him with an impostor.”