BERLIN — When a neighborhood group on a walk through an overgrown patch of land in central Berlin came across a 20-meter stretch of concrete sprayed with graffiti and covered with vines, nobody was sure what to make of it.

“We started debating and someone suggested that maybe it was part of the Berlin Wall,” said Ephraim Gothe, a city councilor for urban development in the Mitte district of Berlin, who was leading the group on a tour in early June.

He alerted the authorities, leading to confirmation that the group had discovered an original piece of the wall’s outer defense perimeter, which prevented East Germans from approaching the principal barrier, near the Chausseestrasse border crossing.

The 65-foot piece of the wall went unnoticed for nearly three decades, largely obscured by the construction of the new seat of Germany’s Federal Intelligence Service, the B.N.D., said Gesine Beutin, a spokeswoman for the Berlin Wall Foundation.