system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are scientists able to say that a random species is extinct? Did they have to look everywhere?
gpt
Its more they check for long enough and find nothing.

The fact that we know about them in the first place, and can track declining numbers for some, makes their absence the basis for that decision.

That said, some species turn into hide and seek champions, showing up long after they are "presumed extinct"