I watched a TV documentary on Sian last week and I'm just relaying the simple way it was explained by the programme.
Here's an interesting article from the New Yorker about cell phone data in relation to criminal cases. The law stuff is American, but the technical info should apply:
What Your Cell Phone Can’t Tell the Police
Apologies
I just think people are using "ping" and actual data/voice connections interchangeably
A simple example. At our place we are equidistant between 2 4G towers aligned on the autobahn. However due to topography, one is stronger and is our home tower and I orientate our outdoor antenna to the east tower. When i turn on our router, both towers "see" my router. However we connect to the east tower not the west tower. This is how the network understands that my router is now on, and how to allocate 4G resource to it. The east tower allocates resource, the west tower does not.
However in the case when the west tower has an issue and I cannot connect to it, my router reconnects itself automatically to the east lying tower. I sometimes notice this when my 4G gets super slow.
All of this network activity is different to when my router is actually requesting data from the tower, or a call is inbound, or there is push data (e.g email, chat etc).
Now the same is true of my mobile, except more complicated.
If i walk west on the property, as signal strength decreases my phone will look to the closer west tower in preference and switch connection. It knows what towers are around, and finds the best one.
So there is a lot of potential ping happening, for someone driving in a car, with a mobile on, even with data off.
And if he had satnav on - was all his data of?
I can imagine him turning his phone off at some point, if he decided to attack libby, but did he have the foresight to do it from pickup?
Maybe not