At the beginning of Season 4's Commencement when Lana and Genevieve were fighting in the Talon apartment, Isobel-in-Lana stabbed Genevieve to death. And Lex shows up when Lana has blood all on her hands. Lana claims it happened so fast and didn't know what happened and Lex told her that she did what she had to do to defend herself. Seeing that Lex is aware that Isobel occasionally possesses Lana's body, why didn't he (and Lana, for that matter) put two and two together and consider the fact that Isobel killed Genevieve, not Lana? Especially if Lana didn't remember it happening. She and Lex should've figured this out.