Bad wording on my part - Ash's tracker certainly tracks motion too, but that isn't what it keys on, as you know.
That said, are you certain it stopped tracking the alien when it stopped moving? All I've got is that Lambert tracked it when it came on the scene, and then we see Dallas's blip by itself for a minute (but are we seeing the whole screen?), and then, "oh god, it's moving right towards you!". I'm gonna have to go look again...