In ODEON any surface is represented by a closed loop of consecutive points. Subsequently, each point is defined by the 3D coordinates: x, y, z, as you can see in any room par file opened in the ODEON editor.

When a window is placed in the middle of a wall or when there is a hole in a surface, ODEON has to create one loop of points both along the boundary of the wall and the boundary of the hole. An extra line appears in the wall inside ODEON giving the indication that the points defining the hole are connected to the points defining the boundary in the same loop. So, this line is an indication of a healthy imported surface with a hole inside.

You can see the loop of points for any surface very easily in the 3DView, by pressing the N key.