So something you could do is that you could make it so it only searches (moves the ultrasound sensor back and forth) when the target is no longer present and once it finds it it moves to it and follows. Not sure if what I am saying makes sense but it will make it use a little less processing power in the long run (built robots all through high school for competitions, processing power becomes important in those situations)
Just a thought.
I understand, it makes sense. I never thought about that, but if we have moving target, then it will lose some time to locate it. By constantly scanning in ±60 degrees like now, it keeps object always in sight so it have continuously following.