A person who has caused harm to his fellow must ask forgiveness even if the victim is unaware that he has been harmed.
A case in point is where an employee was being considered for promotion but was never informed of this possibility. If someone violated the laws of loshon hora and made a statement to the employer which resulted in the promotion not being granted, he must approach the victim, explain what has happened, and beg forgiveness.
However, Rabbi Yisrael Salanter is reported to have held that if relating the incident to the victim would cause him anguish, one would have to forego that aspect of the teshuvah process.
