Isn't it all just a software problem? Build a gizmo with high sensitivity to electric fields, and some decent directionality. Add in very good motion sensing, so that it can tell which changes in its input are due to its own motion and orientation changes. Have lots of RAM to remember the full sensor values at high frequency for the last couple of minutes. Invent lots of computer algorithms to weed out unwanted signatures (power lines, signature of operator, signature of the device itself and other devices known to be nearby, etc.). More algorithms to dig out the patterns resulting from all sizes of targets. Display the targets discovered by this. Simple. :-) You might have to wait a bit until Intel/AMD/IBM can provide a Teraflop or two in the gizmo.
Hmm. With smart enough software, it likely doesn't need the motion sensing. It can display its results relative to itself, just like the Sci Fi ones do.