Hi! I and some friends recently saw armed forces walk on the streets carrying a device that detected guns (and it actually worked – some people got nailed) in houses made out of brick and/or cement block. I and some people who watched it discussed later about how we thought the gadget worked. We never came up with a plausible explanation. We don’t have the faintest idea of how it works. If it was magnetic… well there are a lot of metal objects in every house; if it worked sniffing some kind of gas, as some proposed, how would it smell a gun that hasn’t been fired in a long time? And wouldn’t a dog be better? Well, we don’t know. So I’m asking.