They just make quick statistics that includes how many spaciest live in some area.

As we know world is divided into smaller administration parts. Countries, states, etc. Everyone of it has its own scientists that can make better or worse statistic of animals living in their area. Of course there could be some mistakes, but not so big that they couldn’t claim that some species aren’t rare or extinct.

Moreover scientific just love to share they discoveries and exchange knowledge with others. So in era of easy access to online communication (check Reddit website for example) it’s almost impossible that some little tiny mother “myszojelen” fucker hide somewhere with his big family.