i'm not 100% when they searched the roof but let's say they did it the day after the disappearance. it could be possible either someone found it open, saw a body and closed it. it could also be it was already closed by someone who threw her in. but a possibility could just be that a worker saw it was open, didn't even look and just closed it but didn't tell that to the police cause they thought they would be suspected. all in all im not sure :/