I see what you are saying, but they don't expect you to go through every system to see if you can find something they didn't bring up. All I did for ROS was the same set of questions for every patient - fever, weight loss, fatigue, n/v, diarrhea, constipation, etc... - real general things. All of this in addition to HPI and you should have covered just about everything they expect you to cover. Then at the end of my HPI, I would skip a line and write "ROS: Patient denies x,x,x,x,x. Admits to x,x."
You might be fine without it, but we were told at our school to make sure to cover it. Good luck! 😀