This is an interesting video posted on YouTube in February 2012. Seems like I would have remembered this technology if I had come across it previously. Check out the description of the technology which I will quote in detail:
"Uploaded by arronlee33 on A covert, perpetually self-powered wireless sensor network developed by Lockheed Martin could revolutionize how to discreetly monitor one's surroundings. Lockheed Martin's Self-Powered Ad-hoc Network (SPAN) system is a network of "Field-and Forget" ground sensors that provides unobtrusive, continuous surveillance which can support a variety of missions and applications, such as border protection, area surveillance and even bridge, pipeline, aircraft and other structural monitoring requirements.
SPAN is a mesh network of self-organizing, self-healing sensors. Information from this mesh is processed using proprietary algorithms that reduce false alarms, providing intelligent situational awareness for military, border patrol and structural monitoring applications. This smart sensor network can cue a camera or unmanned aerial vehicle to further study an area or call an engineer when a pipeline or bridge structure is in danger of fracture.
Camouflaged and Easily Concealed
With sensors small enough to be fit into one's hand, SPAN sensors can be concealed in camouflage housings, such as those resembling rocks, to be inconspicuously positioned throughout an area.
The SPAN system also operates with extremely low power, thereby making it possible to power it via energy-harvesting technology, such as solar panels. SPAN's innovative power management harvests energy from its surroundings to provide perpetually powered sensors. Its ultra low sensor cost is predicated on the fact that each node within the SPAN network incorporates an energy harvesting subsystem that re-charges itself using simple energy sources in its surrounding environment. This innovation negates typical life and cost concerns of batteries and reduces the manual deployment and servicing. Battery life can be a critical differentiator when determining the safety of those in harm's way.
SPAN's lighter power demand extends operational range, and its inconspicuous sensors reduce the likelihood of discovery and tampering, increasing the realization of persistent surveillance."