Batwoman did not really like sewers. Unless an old friend fled into one, she would only go in for a little fun during the chase.
Most of the time, she was fairly hands-off with this area, because the figurines also needed a safe zone where they could discuss plans together or secretly hatch schemes.
It was like playing tag, where you marked out a safe "home" for the runners. Once the children escaped into that area, they could no longer be caught. Same principle.
Given Batwoman's wealth and the manpower she could mobilize, she could have installed undetectable surveillance equipment throughout every sewer in Gotham City. But she had not done so, and there was naturally a reason for that.
It was her own subjective reason, not because of any objective constraints.
Today, as she entered a sewer project from a hundred years ago—or, more accurately, a mine shaft—she saw the original state of the sewers for the first time. This network of tunnels that ran roughly parallel to the ground was not what she was looking for.
Nominally, the Wayne Family's ancestor had contracted the sewer project to benefit the townspeople. In reality, it was to secretly excavate the treasures buried with dead indians and fill the pockets of the four great families.
But that was only the second layer of cover. Edmund used those valuables to deceive the other three families while carrying out his own secret plan: laying out the Final Laugh.
In his design, the Final Laugh had been a secret measure meant to perish together with the British, who might cross the New World at any moment. But in the end, after the secret modifications of every generation of Wayne family heads, it became a terrifying Biochemical Weapon—and The Batman Who Laughs had nearly gotten