This page of the Final Fantasy 7 Remake guide has a description of Discovery - Inside the Ventilation Fan. You will learn how to get the hidden object behind the fan.
To get this item, you must first restore power to the Cargo Platform. In other words, you have to turn off three big lights and redirect their power to the elevator. You will do this by following the next mission objectives. Once power is restored, go to the platform next to the main elevator and go further.
Sit on the bench to heal and then use the panel. You have one minute to eliminate enemies in the next room. In your first try, you will face Queen Grashtrike with one monster. However, if you don't make it and try again, you will face regular enemies. Deal with the danger and quickly activate the panel next to the door.
That way, you will turn off the fans. You can now grab your reward.
Reward: Summoning Materia - Chocobo & Moogle Materia.