☄ The tunnels aren't empty, but the only people that know that they exist are slaves and the people that help them escape. If he explores long enough, he'll run into a group trying to move across the city and if he explains he's not any trouble, they'll tell him that they're trying to get out. These tunnels are not known by many and are used as a way to take slaves from their owners and sneak them away to somewhere safe.

NOTE: Your character can use this knowledge to do the same or for other reasons.