Sarah and Reese are in the factory. The Terminator somehow homes in on them, but what factor allowed it to do so?
The only giveaway is that the Terminator's pupils dilate and then normalize when Sarah accidentally trips the switch on one of the machines and the sound is heard by the T-800, but how was that enough?
The film's script make it clear that the Terminator was listening to the factory machines. When Sarah set off the hydraulic press, that focused its attention on a new sound, followed by her stumbling onto the catwalk, a "non-rhythmic" sound that make it think that it might be her.
Her knee inadvertently hits a RED PUSH BUTTON. With a ROAR the stamping-plate of a HYDRAULIC PRESS slams down an inch from her hand.
Startled, she tumbles to the catwalk.
The Terminator's eyes swivel as he hears the single non-rhythmic sound.
How he was able to hear her over the cacophony of sounds is addressed in the film's novelisation. When you see it turning its head back and forth, it's excluding the sounds of the factory machines from its awareness, leaving only the sounds of vermin and its target.
With a sudden roar, the stamping plate of a massive hydraulic press slammed down an inch from her hand. Startled, she tumbled onto the catwalk.
Terminator’s auditory sensors had filtered out all arrhythmic sounds and identified their patterns as vermin, water dripping from bad pipe fittings, and the target moving in the machinery up ahead. It swiveled its head on precision bearings and moved toward the sound.
