Welllll ... I found it. But not in the couch.
While taking a breather, I searched for the rest of the set to show my husband what this whole escapade was about. I found the set where I remembered putting it three years earlier -- in a shelf, in its original mailer, which also contained an open Chinese Checkers box full of Japanese pinchers which I had acquired at the same time from the same seller. And nestled on the edge of the green row of pinchers was the missing green fiber optic marble. I never did drop it in the couch after all. I dropped it back into the mailer. I only imagined that it dropped into the side of the couch and I only imagined that I saw it dropping further down as I reached for it.
At least that's what I'm telling myself.
My husband assures me there is no such thing as poltergeists who find lost marbles and return them to their mailers while I am not looking.