Maybe the difference between the Dodo and the Christmas Island Shrew is 350 years.

Which is to say, the certainty of the Dodo's extinction is related to how long we've not seen one.

Every year that passes then without the shrew will be to underscore its extinction, I suppose. Sad.

I have a feeling that it depends to some extent on the organism. It would be hard for a breeding pair of Dodo's to hide for very long, whereas a colony of little shrews could creep around in the bushes without notice. "Last seen on X date" would cover both cases.