Adrian openly threatened Cecilia that should she ever leave him, he would be able to stalk and haunt her without her ever seeing him, and he very much made good on that promise. Using his incredible resources, and having his brother as an accomplice, the psychopathic inventor successfully faked his own suicide, and used it as an alibi to literally disappear. Finding out that his estranged wife was living with James and Sydney, he made a nest for himself in the attic of the house, and used it as a base of operations for gaslighting Cecilia.
