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To answer my own question: I remembered again this type of code: It is linear regression. The basics is that the wall is assumed to be straight and that the laser scanner or Kinect projects within a sufficient small window a number of scans with points (range, angle) on a straight line y = a.x + b. These polar coordinates are converted to (x, y) coordinates and inserted in a linear regression (least squares) formula. We are interested in the direction of the line, or better the perpendicular line (that is rotated 90 degrees). I think the code snip above is mixing the cos and sin as for a right-handed coordinate system the normal definition is x = r.cos(alfa). Because of the perpendicular line the final result is ok after all.

For a line you need of course at least 2 scan points, and for a reliable result many more. Perhaps you also need to average the angle for less noise.