It's not talked about it much outside of research groups, but for any field, there is a small number of people who are currently pushing the boundaries, and they all read each other's papers and have a good idea of what the next question to ask is. It can often be a race to engineer an experiment that convinces the reviewers that your article should be published first. It's a sort of cooperation/competition that moves the field forward faster. These areas often move so fast that nobody even bothers to write down the current problems, it's just sort of talked about in person.
Put another way, the successful discoverers are the ones looking for their keys at the end of the streetlight: "Science is a bit like the joke about the drunk who is looking under a lamppost for a key that he has lost on the other side of the street, because that's where the light is. It has no other choice." (Chomsky). Few if anybody looking where there is no light discovers anything (even if it's sitting there in the dark), or at least, nobody believes them unless they provide significant evidence (like building a new lamp)