How do you suppose the fire at Manderley was set?

The book never explicitly says that Mrs. Danvers set the fire but it is implied. Maxim calls the house from the hotel in London that they are staying at after going to meet the doctor and discovers that Mrs. has cleared out. Some removal men had come to collect all of her boxes and nobody could find her, but nobody had seen her leave either. Then Maxim and the second Mrs. de Winter have this conversation which foreshadows what is to come and implies the involvement of Mrs. Danvers and, most likely.