Or maybe they are used for fraud. Tunneling traffic through a sim card nobody knows about, which if located would be discovered in the mountains, well that's a blackhat dream
True but someone has to purchase the sim and top it up - money trail, plus there's physical evidence where did the box, solar, whatever's in it come from serial numbers etc more money trail as well as physical evidence to worry about. Plus - how long as it been there - what day did it come live - cameras covering trail enterances, nearby gas stations, what other phones pinged the mast that day etc. Plenty for the white hats to go at there.