I watched a Dateline episode recently in which cell phone pings played a big part.
The killer took a guy out on a boat, shot him and dumped his body overboard. He did such a good job on cleaning the boat, there was no evidence of what took place. However, LE did look at the pings on the two phones. If you mapped them side by side, they traveled to the exact same spot at the same time, and came back the same way.
I know Erin's phone was turned off, but we certainly don't know exactly when. IMO, we are going to see some evidence similar to the above. At least on the way out to the mine, and briefly while they were actually there. It's certainly not a far-fetched conclusion to arrive at, MOO.