Don't feel bad on failing the Spot.
I have it on good sources that the thread, being a magical entity (just look at the title), has the Invisibility spell, or a similar ability.
I reccomend that you view these baords with Goggles of See Invisibility or a Gem of Seeing to prevent similar incidents in the future. Blindsight proves inadequate for dealing with invisible characters on a flat computer screen, for obvious reasons.