I agree, with respect to (e.g.) math. People reason that "someone would have noticed" implies that there is no undiscovered low-hanging fruit in math.

My skepticism of this conclusion is based on my perception of how mathematicians work. They are fairly autonomous, working on the things that interest them. What is interesting to mathematicians tends to be the large problems. They swing for the fences, seeking home runs rather than singles.

Plus, there are unfashionable areas of math. A consensus that certain areas of math have been fully explored (nothing new remains) has developed, but not in a systematic way. So, it's not clear whether this consensus is accurate, because politics (for lack of a better term) were involved in its formation.

It's only reasonable to be confident that 'someone would have noticed' if someone knowledgeable about what they are looking at actually looks in that direction.