@cicerone imposter,
Well, not all socks are happy with their job. Sometimes one sock is more sensitive to dirty feet than its mate is and will flee the situation. If your feet are not dirty and do not smell bad, then it is likely that one sock did not get along with its mate, and so they divorced. As to the question of where they go, all that can be said for sure is that they do not wish to be found. And that is their right.