Machine quilting on I Spy Quilt #1

Here is our first (of this batch anyway!) I spy quilts quilted. I used a variegated primary color thread and just quilted swirly pinwheels in the yellow blocks. I did the same in the border. I don't usually quilt in the novelty prints. How did that "quilting thread" square get in there?

The yellow thread blends into the background and seems to disappear from view.

~Joan