Back when this was first reported, I also wondered if this was even possible and wrote to Apple Support with the following question:
"If a fully charged IPhone were to be left outside, for example, beside a road overnight with temps dropping to mid 20's (daytime highs of mid 40's) :
1) would there be any battery life left and
2) could phone be located 24 hours later via any Find iPhone app? (can phone be located if no battery life?)"
Their response:
"Under ideal circumstances, it's possible the phone could still have small amount of charge if fully charged at time it was left/dropped outside, but would depend on condition of battery and apps loaded onto phone by user.
You can only locate your device when it is logged into iCloud and 'Find My Phone' is enabled, additionally the device will need to be switched on (and charged) and connected to a wifi or cellular network.
Unfortunately, you cannot activate iCloud or 'Find My Phone' remotely.
Moisture/condensation may affect the phone response/battery life if it's left outside."
My take was that because we didn't know how much charge Jen's phone had at the time she "left the house", or what apps she had on the phone that would drain the battery more quickly, or the condition of her phone battery, there was no way to tell absolutely. I did base the question on the weather conditions the night she disappeared.
I guess I should have also asked how easy it would be for a rock to turn on the phone..........