How does the motion tracker in "Aliens" work? What's the technology behind it (hypothetically)?

Question

How does the motion tracker in "Aliens" work? What's the technology behind it (hypothetically)?

Answer 1

Could be gravitational, sensing the "gravity waves" (which are as yet unmeasured) sent out by mass moving around.  There would be no way to mask that, and we know gravity penetrates pretty much everything so it would work through walls and so forth, so it would be a good thing to sense for motion if you could.  

Radar or other electromagnetic-based method is improbable since EM in general wouldn't penetrate a metal-framed building.