Trivia: The name of the demon, "Valak" can be seen on the playground equipment where we first see sister Irene.
Trivia: You can see the word Valak spelled out on the licence plate of the truck that drives off with the luggage.
Trivia: The Demon's name "Valak" can be made out in the reg plate on the truck that drives away with Father Burke's luggage.
Trivia: The name of the demon, "Valak" is visible on the registration plate of the truck in the village just before Sister Irene, Father Burke and Frenchie leave for the abbey.
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.