Without releasing too many details, the U.S. Department of Defense announced that they are building disposable sensors that use RFID technology. RFID is an electronic device that, when hit with the right electronic signal, responds with information contained in it. Right now, RFID is being used for inventory control. Cargo containers, or individual items (like a can of food) with an RFID chip (they are really cheap) attached, responds, when a hand held RFID reader nearby broadcasts a signal. More expensive RFID chips can respond to a reader farther away, say like one in a low flying UAV. The Department of Defense is building motion sensors into fake rocks, small (golf ball size) fake rocks. The rocks will be equipped with motion sensors that can detect footsteps 5-10 meters away, and various type of vehicles moving nearby at ranges up to a hundred meters. These sensors can be dropped from the air, or placed near American troops (who will then monitor what the rocks hear via automatic RFID interrogators placed behind the rocks.) These sensors are cheap enough that they dont have to be retrieved. Such sensors enable intelligence troops to monitor enemy movements over a wide area. With these smart rocks, troops in Afghanistan can, for example, regularly monitor the many mountain passes used by hostile forces to sneak in from Pakistan. You could do it in real time, by dropping a large repeater rock to work with dozens of smaller sensor rocks. The repeater would regularly poll the small rocks, and transmit the data back, via a satellite link, to a base. The repeater rocks would have to be replaced as their batteries wore out. The smaller rocks would also have a small battery to store data, and time it was picked up. But the batteries in the small rocks would probably last for months. 

Sensors like this are nothing new, they were available, in cruder form, as far back as the 1960s. The new generation of remote sensors are, however, cheap enough to use on a wide scale. Hollywood fake rock technology is probably being used as well. Movie makers have been producing more convincing, lightweight, fake rocks for decades. These rocks come in many shapes and textures, but are actually light weight and made of a rubbery material (so actors wont get hurt when they are hit with a bunch of them). This also makes it possible to drop the rocks from the air without damaging most of them. Dropping the rocks, at night, in remote areas, means that they would be almost impossible to detect.