Regarding the comments about the light only coming on when the door is locked, I am certain that this is the intent. However, this is not the case in practice. The lights often come on when the lock is closed about 50-75%. The sensors must get worn out over time. So maybe she thought the door was fully locked because the light came on and the FA did not see the sign saying it was occupied. I know that sometimes the sign is halfway and can indicate that someone is inside and didn't fully lock it, or that someone didn't fully unlock the door before leaving.