This is more of a question than a comment, or maybe both and if somebody knows how all this actually works, I'd be happy about an explanation. This, of course, only works if they know the phone number. Or at least the phone's IMEI number (which stays the same even with a different SIM card, it's unique for every cell phone, like the DNA of the cell phone, if I understand correctly). What about WiFi? Not saying that there is WiFi in the park, but probably in other places and people also have it at home (I don't, so that's why I don't really know how this works). But if you know what phone or phone number you are looking for and someone is moving with the phone, the phone will also log in and out and back in again at different WiFi's along the way. Which basically shows exactly where the phone is (or was). Just thinking that this might have been another way that they could have checked cell phones that were in or near the area at around the time of the crime, before and after. Or even the next day. Plus, police can send silent SMS and track a phone that way (but again, you need to know which phone/number you are looking for, that is more something that they could have used to track Libby's phone, but I guess they just found it near her somewhere - but they could have tracked it with this if the monster would have taken her phone with him). All MOO and as I understand it.