L probably did not notice any of it because 1.) He did not automatically remember her name, and 2.) If L would have found that and saw that Light was the last one with her while she was alive, he *could* have arrested him, but they were in a *big* city, so they wouldn't have had evidence that it was **Light** that killed her. Also, if L *did* arrest him, the entire story would have been shorter and not as fun as it is now.