Sarah recalls to Kyle how her parents were killed by a T-1000 sent to 1973 to strike at her when she was just 9 years old. Luckily for her, the reprogrammed 'Pops' T-800 is sent back through time by an unknown member of the resistance to save her. What became of the T-1000 isn't directly explained after Sarah is rescued.
The T-1000 that was sent to 1973 is the same T-1000 that appears in 1984.
This is why we don't see the T-1000 undergo a Time Displacement Event in 1984 as did the original T-800 and Kyle Reese. It must have concluded that the highest probability to reacquire Sarah would have been to intercept Kyle Reese on the day of his arrival and mimic his appearance.