now entering the French sector? (spoilers)
The tunnelers made a sign marking the border between French-occupied West Berlin and East Berlin. Matthis the engineer and the rest of them would have known exactly where this was, but they could have placed that sign anywhere arbitrarily. Why did Colonel Kroger trust the accuracy of the sign? The tunnel, I presume, was illegal in both West Berlin and East Germany.
I wonder, what was the East German protocol for retrieving illegal emmigrants from cross-border tunnels? How could anyone but Matthis really know where the Berlin Wall was?