Other mistake: Father Burke is buried in a coffin that is not air tight. Light from his torch can be seen outside the coffin. Yet no dust or dirt works its way into the coffin. (00:40:00)
Other mistake: Sister Irene digs father Burke out of his grave in a crisp white night gown but doesn't get any dirt on it.
Answer: The nun was trying to find the relic. The deceased nuns had no actual idea where it was. That's the whole reason Irene was summoned by the Vatican because she had visions about "Mary pointing the way" also the "ghost nuns" more than likely have something to do with her clairvoyance as well.