My cistern is indoors, can it rust?

Indoor cisterns rust just as much as others. Most often, you have a ventilation duct in the boiler room due to the oil burner needs plenty of air to work, usually that valve is located near the cistern. This means that when it is cold outside and the burner needs a lot of air, the cold air flows in over the hot cistern and condensation forms in the air layer inside the cistern.