"Without a doubt, there definitely is one." Angel gave a definitive answer in the Mind Ribbon: "However, this also indicates one thing, there's indeed a way out hidden above the chimney."
The sewage systems and the labyrinth are actually one and the same, now being discussed separately merely for later classification purposes.
Before Naraku City fell into ruin, the underground and the surface were quite alike, both containing large living areas. It wouldn't be an exaggeration to call it an underground city. Otherwise, Naraku City wouldn't have established various official institutions in the Underground Labyrinth.
Wherever people live, there naturally needs to be sewage channels, hence the later "sewage system."
The appearance of mutated Carrion Squirrels on this path suggests that there must be a sewage system, and where there's a sewage system, a living area is not far. A living area implies a way out.
