Does the fasting person who drank, not knowing whether he was conscious or asleep, break their fast?

The default is that the fast is valid. As long as you are not certain that you intentionally drank while awake, your fast is valid, and you are not required to make it up. Assuming the drinking occurred while you were asleep, it appears that you did not do it intentionally due to being overtaken by sleep and not having your full awareness. Consequently, your fast is valid. One of the conditions for invalidating the fast is that the accountable person intentionally invalidates it; otherwise, their fast is valid, and no make-up is required. The pen is lifted from the sleeper, and their action was not by their choice, so their fast is not invalidated, and they are not required to make it up.