This is just my own speculation, of course, but when L captured BG walking on the bridge, I don't think it was overtly obvious to him that he was being filmed. In the the DTH podcast that has been linked several times lately, LE tells us the video is 43 seconds and "G...DTH" is at the very end. It therefore doesn't seem there is footage of anything after that on L's phone (that we know of). He orders them down the hill and the video stops. She could have dropped her phone right there, near the south end of the bridge, or at any point between. Only LE knows where it was found. It's nice to think L left the phone purposefully, as a bread crumb, but my feeling is that she still didn't know at that point that she was going to die, so she likely would have kept her phone on her unless she dropped it by accident or during the crime itself. Who knows, the killer maybe didn't even think to take it, or he knocked it out of her hand to keep her from trying to call for help. We just don't know, but I don't think the phone was of any significance to the killer in that moment, because he didn't know he was on it.