It was true that Walter didn't like Leah. Why should he? She was an assassin. She came to Los Angeles just to kill Cabe. So what if she hadn't? He was supposed to praise her for being a decent human being?
He had wanted to talk to Cabe about her, but six months later they were dating.
Walter was looking for dirt on her. Something that would convince Cabe that he had made a mistake before it was too late. Walter didn't want Cabe getting hurt.
Leah couldn't be trusted.
He'd keep looking.
He was willing to wait for it.