Go down to the first floor with the ladder and go to the second floor in the elevator.

Go down to the first floor again with the ladder and move to the elevator door.

A rear door can be seen if you open the elevator door.

Check the right side and get the round part on top of some sort of device.

If you check the area around the bottom of your feet, you will find that a part of the wall is broken. As the inside is dark and nothing can be seen, light a match to check the inside.

The flame will move as you drag it so drag it to the left to get the fallen key.

Move to the wall with the wooden door. Enlarge the wooden box placed on the floor on the lower left.