Rats do not live in a working sealed drain by choice, but a cracked pipe or an abandoned connection left open during a renovation gives them a route into the system and out again into gardens and under floors.
Signs of rats in drains include scratching noises in walls or under floors, droppings near chambers, and burrow holes appearing near the drain line.
The permanent fix is finding and sealing the defect — a CCTV survey followed by a patch liner or a non-return valve on the affected run. Baiting alone does not close the route.