One morning last July, the German intelligence service knocked on Daniel Bangert's door. They had been informed by the US military police that Bangert was planning to stage a protest outside the Dagger Complex, an American intelligence base outside Griesheim in the Hesse region. Why hadn't he registered the protest, and what were his political affiliations? they asked.

Bangert explained that he wasn't planning a protest and that he didn't have any links to political groups. All he had done was put a message on Facebook inviting friends to go on a "nature walk" to "explore the endangered habitat of NSA spies". Eventually, the agents left in frustration.

Twelve months later, Bangert's nature trail has not only become a weekly ritual in Griesheim, but also the frontrunner of a new multi-generational German protest movement against digital surveillance.

On Saturday, around 130 "spy spotters" from across the country joined Bangert and his "Society for the Protection of NSA Spies" for the first anniversary of his ramble from Griesheim's town square to the Dagger Complex.

The 29-year-old heating engineer, who is currently retraining as an IT worker, first became interested in the US intelligence service after the publication of Edward Snowden's revelations in June 2013. When the NSA advertised a job for a security expert in the area, it seemed to confirm his suspicions about the fenced-off site in his home town.

"There had always been rumours that the Dagger Complex was full of US spies when I grew up", he said. "The incredible thing is that those rumours have now turned out to be true."

According to reports in the German media, the Dagger Complex is the central base for US surveillance operations in Europe, containing the NSA's military branch as well as the "European Cryptologic Centre" in which several hundred NSA employees collect and analyse data with the help of tools such as the notorious Xkeyscore programme.

Usually, Bangert's spotters carry at least a pair of binoculars and a couple of surveillance cameras made of cardboard. Registration numbers of cars parked at the facility are logged, but actual sightings of NSA employees are rare: "Spies are shy creatures," Bangert said.

On Snowden's birthday, the walkers brought along cake in order to "lure the spies out of their hiding holes". When the owners of the Dagger Complex complained to police about the rubbish left behind by the protesters, they brought rakes and brushes and offered to clean up inside the complex.

For the anniversary, the organisers built a "bed for Snowden", as a symbolic reminder of their ongoing campaign for Germany to offer asylum to the US whistleblower.

So far, Bangert's only interaction with those working in the Dagger Complex is the time a departing employee wound down his car window and called him a "dumbass motherfucker".

"My aim is to get on the NSA's nerves whenever I can," Bangert told the Guardian. "And I think they are pretty irritated at the moment."

The protesters include students in their late teens as well as pensioners. Nadine, a 20-year-old from Lower Saxony, has been joining the protest once a month since last autumn. "Most people prefer to complain from the comfort of their sofa," she said, "but here people are actually doing something to voice their complaints."

Frieder Haug, a 67-year-old retired priest who is also a member of the activist network Attac, said he joined the walks because he felt the protests against digital surveillance had managed to bring together different people in the way the peace movement had in the 1980s.

"Originally, I didn't want to join Attac because it was only full of people my age. Now there is finally a new generation of young people who are not only politicised, but also stubborn in getting their point across."

Bangert said he planned to continue the nature walks indefinitely. "Over the last 12 months, in spite of all the politicians expressing their outrage, absolutely nothing has changed. So of course we have to keep on going".