As morbid a thought as this is, maybe Charli lost her phone in the melee. Maybe she threw it at the last minute so her family would be able to find her. Could be SC completely forgot about her phone until he texted her that night (a text that may have been his attempt to find her phone!).
Yes possible it just got left in the jungle there. Or since he hid her SUV a while not hard to think he could hide her phone too.
In first days searching was reported her phone was still on, ringing. Phone location didn't ping again on app she had on her iphone so app was turned off or just didn't update if phone didn't change location? Thinking about the phone because it's still missing and if he had control of it what did he do with it. And her laptop is also missing.
Easy to think because her skirt, blanket, dvd, other stuff were found there her phone is there too. Seems like he had a plan for the laptop though so I'd think he had a plan for the phone too.