What is the ruling on one who doubts having committed a sin and repents from it—will they be pursued by intrusive thoughts despite not being certain of the act?

General repentance is prescribed for the Muslim always, whether he remembers a sin or not. It is prescribed to frequently say: "I seek forgiveness from Allah and repent to Him," based on the Prophet's (peace be upon him) saying: "O people, repent to Allah, for I repent to Him one hundred times a day."