|The following is based on Fallout Tactics and some details might contradict canon.|
Echo Boy, or EchoBoy is a piece of technology used by the Brotherhood of Steel for surveillance and reconnaissance.
There is very little information about the nature of Echo Boy. However, it is known that the technology makes it possible to detect large concentrations of metal. The foregoing suggests that it is of electromagnetic nature.
Another guess would be it uses Sonar technology much like bats, where an ultra high frequency sound bounces off a large mass of metal. Much like a submarine.