Having a card up your sleeve is still valuable, even if nobody knows it is there, because it means if you have to pull it out all of a sudden, it's there. Taiwan would be my A#1 pick for "country most likely to secretly have nuclear weapons" because while everyone around China has reason to be nervous at one level or another, Taiwan's clearly got the best one.
"The world" as a whole is also a bit jumpy about nuclear weapons, so pulling the card out early isn't free. If for the sake of argument Taiwan does have some prototypes lying around, they must have judged the risk of a reveal greater than the reward. But that can change swiftly.
One of the questions I find most interesting, from my civilian and total outsider position, is: How developed are the world's intelligence agencies? How good are countries at successfully keeping things secret from each other? Is it theoretically possible for Taiwan to have secretly developed a prototype bomb without China covertly knowing about it? (This is an especially interesting question in that for structural reasons, Taiwan and China have means to spy on each other relatively easily.) Is it in fact the case that everybody basically knows everything about everybody, or are there many, many successfully kept secrets? What are the probabilities of discovery? What's the appetite for risk of discovery within the relevant governments? I expect from my current position that it is simply impossible to get solid answers to these questions. It also seems to me that while the world does not care for countries overtly advertising they have a bomb, the game theory for covertly making it known to certain of your enemies that you have the bomb may be quite different.