

Look up microphone array sound cameras.
Obviously range will be limited proportional to width of the array and sensitivity and ability to filter noise, but a few km is plausible with a military budget (but you’re not really going to be able to identify individuals without backing it with imagery, and resolution through buildings will be almost none)
Edit: also, heat cameras are way simpler/cheaper for this task unless you’re looking for somebody who knows how to cover themselves in a silent area













If they’re detecting it based on electric fields they absolutely won’t be able to pull off usable range outside of an area with absolutely no big collection of metal (can not be used near buildings or roads)
Typical range for sensors that can pick up human electric fields (mostly generated by muscles and nerves) is a few meters. If you try to boost it enough for long range outdoors use then your sensor is gonna get screamed to death by the sun and atmospheric noise