In the shooting script we learn that she heard crying.
Ripley running toward engine room.
Out of breath.
Exhausted she stops, gulps for air.
Suddenly, ahead of her, the sound of human weeping.
She moves quietly ahead until the source of the sound is directly under her feet.
She is standing on a round metal plate.
Ripley starts to remove the disc.
The official novelisation contains much the same description.
All that thinking brought her back to her senses. Gulping for breath,
she slowed and took stock of her surroundings. She'd run the length of
the ship. Now she found herself standing alone in the middle of the
She heard something and stopped breathing. It was repeated, and she
let out a cautious sigh. The sound was familiar, human. It was the
sound of weeping.
Still cradling the flamethrower, she walked slowly around the room
until the source of the noise lay directly below her. She found she
was standing on a companionway cover, a round metal disc. Keeping half
her attention on the well-lit chamber surrounding her, she knelt and
removed the disc. A ladder descended into the near darkness.