A new approach to car alarms gets vehicles to watch each others' backs like a herd of animals under threat from predators.This is a great idea. It relies on strangers to help accomplish a positive goal. The basic idea is that when parking a car, the car joins an ad hoc network of other parked cars. Any car that moves without authoritatively signaling that it's going to move causes the other cars to raise an alarm. See the details in the story or the original paper.
The security system relies on networks of cars constantly gossiping with their neighbours using concealed wireless transmitters. The cars raise the alarm when a thief tries to make a getaway with any of their number.
What I like about it is that it relies on the overall goodness of strangers. It assumes that when one parks, most of the other cars will perform as expected. Society depends on that in general. Society doesn't work because the police keep us all in line. Society works before for the most part people are honest. If there are more honest people than dishonest people a society will work. Otherwise it won't — at least not in a way that we find comfortable. (There's a nice agent-based model about that, but I don't recall the exact reference.) A similar strategy is why some of the the wisdom of crowds effects work. It's also why it's better to be out in a crowd than out alone.
So this technique takes advantage of general honesty to let things (which are proxies for their owners) protect other things.