Now, if it was just one piece, I might think that those cords are places that the metal thread has fallen away, or been removed. However, there are a number of pieces. Certainly metal thread that has fallen away would not fall away that completely on all of the pieces.
It does still remain possible that the metal thread was all deliberately removed, and then it would be all gone. There are other reasons I think not, however. Note the halos .. there is nothing there, but there are some of the couching threads still around the circle. This is particularly easy to see in the piece titled The Ascension. Another factor is that the cords that are present are in very good shape. If these were cores of metal threads, removing the metal would have left some damage to the cords.
Another think I love about these pieces is that they appear to be in various stages of completed. The designs have been printed or drawn on the ground fabric. Some, like Christ Carrying the Cross and The Baptism of Christ are missing a lot of the embroidery. From the "crumbs" left, it looks like much of it has fallen away. Some are more complete, such as Presentation in the Temple, where many of the colors are still bright and clear.
Overall, a wonderful set of embroideries to spur lots of thought, speculation and inspiration.