The PSNI only began its investigation into Eamon Bradley after becoming aware of material on Facebook and in the media, a detective confirmed.
The 28-year-old, from Melmore Gardens in Creggan, is accused of terrorist related offences in Syria in 2014.
The Muslim convert was arrested in November 2014.
He denies the charges, which include being trained in the use of weapons and grenades.
On Thursday, Londonderry Crown Court was shown a photograph of Mr Bradley kneeling behind a "tripod" of AK-47 assault rifles.
A lawyer said the photograph was taken from Eamon Bradley's Facebook page.
On day three of the trial, the prosecution said Eamon Bradley told detectives that the photograph of him kneeling behind the "tripod" of AK-47s had been taken in a watch tower when he was acting as a lookout for enemy forces.
The lawyer said Mr Bradley told police that another photograph of him on horseback and in Arab dress was also taken at the training camp.
He told detectives that fighters from the Islamist faction Jaysh al-Islam (the Army of Islam) had welcomed him "with open arms".
Mr Bradley also told police that his rebel grouping had been targeted in a car bomb attack by so-called Islamic State in Syria, and that although the bomb had not caused any casualties on his side, it did kill the person in the car.
The trial will continue on Monday.
A Londonderry man accused of terrorism walked around the city freely for a week after he returned home from Syria, a court has heard.