Why do you say "definitely not silky"?
If you are sure of which egg, then you know the mother. Either she has a crest or she does not. If she has no crest, then the crest should be coming from the father. If the Silkie is the only crested rooster, then he should be the father.
If you want to check, you could pen up the Silkie for a month or two, then collect eggs and hatch them, and see if you get any crested chicks. If yes, then someone else does have a crest gene (leaving the mystery of why they don't have a visible crest themself.) If no crested chicks in that case, it will be pretty good evidence that the Silkie is the father of every crested chick.
Maybe it would help if you post photos of the possible parents. Someone else might be able to spot some detail that will solve the mystery.
