Dreaming about old, forgotten mummy means that issues will come to light.
Dream dictionary: Mummy dream meanings
(See Sarcophagus)
(Casket | Coffin | Mummy case) A dead person or a mummy inside a sarcophagus in a dream represents unlawful money. If the coffin is empty in the dream, then it represents a house of evil, or an evil person who is sought by people of the same trade. (Also see Coffin)