So I checked the trunkal abyss and sure enough, the stopper was there! Retrieved it with a set of needle-nose pliers, cleaned it up well, and went to go get some glue or tape or something to keep it from wandering off again. Within minutes, or perhaps seconds, the stopper had disappeared. It is somewhere in my house, of that I am sure, but other than that it is MIA.
I must be losing my mind.