What kind of NPC service sims are they?
Maybe coworkers for missing filled jobs?

The game will create some homeless sims no matter what you do, I guess to have on hand should they be needed as instant coworkers or something. I've noticed this in saves where I have everything completely locked down: no immigration allowed through SP or Register and no empty roles that might need to be filled. As long as those sims stay in the background, I don't really care.