He may have just shut off her phone to keep her from using it; we don't know for sure if he remembered about his own phone. If he was so CSI-oriented, why leave the grips there at all, why not ditch them elsewhere or destroy them? And he seemed to have forgotten all about video, which is really stupid these days. Also, we don't know what LE has to place him at WB, but they have something, another something that did not occur to him. I don't think he is very smart at all.
I also don't believe he is a serial killer who has been getting away with crimes for years. He spent most of his adult years in prison and before that, seemed to stick to assaulting women he knew. JMO