Little Sister Vent

There are numerous holes in the wall, surrounded by a metallic fixture. Little Sisters are small enough to fit in these and crawl through them; therefore they use them as a means to safely move around the city without a Big Daddy. The design of the metallic plate, which includes stylized leaves and plant stems, insinuates that the vents may all lead - possibly through a pumping system - to Arcadia, transporting oxygen and carbon dioxide to and fro. Where the vents go and why they were originally built is never explicitly told in the game, nor is it particularly important to the story. What matters now is that the Little Sisters use them as a passageway.

Big Daddies are always awake and always on vigil. When they do not have a Little Sister with them, they roam to the nearest vent and pound on its rim. The noise summons a Little Sister out. When the Little Sister is tired/full and no longer wants to gather, she goes back to the vent and the Big Daddy usually helps her up into it.

If you hypnotize a Big Daddy or save a Little Sister after taking down a Big Daddy, she will scurry back to the nearest vent and crawl away.

On levels where Jack has already exorcised every Little Sister on the level, lone Big Daddies roam around and continue to pound on the vents, but no Little Sister comes. They will show physical signs of aggravation and depression, then trudge vainly onward to the next vent.