An older, 2007, MIT prototype used sonar to locate landmines. It used narrow acoustic beams at ultrasonic frequencies to detect metal, plastic, and other standard landmine types. It was in a flashlight type of shape to be used by soldiers in de-mining operations. The prototype was 100% accurate, with no false alarms. I will look into the latest incarnation of this device.
With up to 70 million land mines still out there, and 26,000 deaths a year, detection equipment is sorely needed.
A lot of this technology was developed by the mining and mineral exploration guys to find oil, gas, gold, copper, etc. I am surprised that it has finally gotten refined for use at this level. I wonder if they have used it for finding land mines yet? I could se a lot of uses for this device in search and rescue, especially in earthquake areas like we have just seen in Japan.
Lets hope that the technology gets commerciallised real soon for those purposes. As for your neighbors, you could always use a radar detector or get a really big dog. Spreading the rumor that you have automatic weapons in the house can also make them more wary than curious.