The FBI (and prob. other federal agencies) have had a device something like this for at least the last 2 years. From what I was told, it consists of a radar "cone" emmiter (similar to the ones in speed radar guns) that operates at a very high freq., and a dish shaped collector that receives the reflected waves and is cabled to a box about 2/3 the size of a laptop with a small LCD screen. I only saw it from about 20 yards away, but it looked to be fairly accurate in "seeing" movement and object density. The female agent that told me what it was said it was a new device they were testing that came from the same lab that perfected ground-penetrating radar that is used to locate bodies in shallow graves.
I would not be surprised if the unit has gotten smaller, maybe even 6 cell mag sized, but you have to wonder where the display screen is. Kind of like the stealth planes, the fed's keep silent on their really cool toys until too many regular citizens have seen it for them to deny it's existance. Bet we don't see one in our dept's for many years.
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