When using about 20 fiducials some fiducials are not detected by the raytracing. The raytracer returns NULL for this object. They are not hidden by another object and also in range. Reducing the number of fiducials has the result that this object can be detected.
The attachment contains a world and a simple controller. Starting the world and moving (drag + drop) the robot (the red object :) ) over the map probably the upper left corner won't be detected. If the robot is initialized on another position other fiducials are not detected.