Ghost Murmur was described as a futuristic CIA tool that could detect a heartbeat from vast distances. Physicists say the public story clashes with the basic limits of magnetic sensing
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
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