Yesterday's BBC showed that a rock-like drop box was found somewhere in Russia, which is actually a hi-tech spy drop box. It looked more like a normal rock in the video and it would probably be a wireless mesh node. It is just my guess. I had only read about some tree hole and some pigeon nest used as drop boxes and that too in Fredrick Forsyth novels.
I think, now, NSA is going to diagnose all the rocks in the United States to check whether they are rocks indeed. Who knows Russians or Al Queda might have come across the same idea to spy others. NSA will first start with the Rock of Gibralder.