This from mumsnet (I hope you do not mind my liasing between this forum and mumsnet, it's just that there is a very active interest over there in this, and people are coming up with suggestions and thoughts, as well as prayers and support)
ChippingIn would like to know whether her mobile phone has been checked up on.
"when she left her phone was working, a few days later it ran out of battery (well it stopped working, presumably ran out of battery). The phone company would be able to tell the police roughly where it was last being used (this might still cover a reasonable area, but it might narrow it down somewhat)."