The IUCN was ready to call this poor critter extinct back in 1996; and if the last confirmed sighting was in the mid-1950s, then today's news from the US agency is a few steps beyond where we are with, say, the Yangtze river dolphin or Baiji, which a Swiss worker in China who had been looking for it for some time declared last winter to be extinct.

The Monachus-Guardian page has a picture of a Caribbean monk seal from 1910, lying beside his/her pool in a zoo in New York:

http://www.monachus-guardian.org/factfiles/carib01.htm.

As for the Steller's sea cow, their last known range, along the Pacific shore of Siberia, is indeed very remote.  But they were very large animals, and usually human observers did not have a hard time spotting them (unfortunately for the sea cows!).  They disappeared in the 18th century; so if one were to be discovered today, that would be an even more amazing reappearance than that of the Ivory-billed woodpecker.

In connexion with the Caribbean monk seal, I was interested to read about their Mediterranean cousin, who appears in ancient Greek art and literature.  Cf. the fascinating little story that Menelaus tells to Telemachus, in Odyssey 4, about how he caught Proteus the Old Man of the Sea, when Proteus had come up onto a beach to take a nap among his beloved seals.

In the Mediterranean and the Caribbean, as well as around the Hawaiian Islands, monk seals seem always to have been hunted.  But nowadays, presumably it is habitat loss that is the biggest threat to them.  And that habitat loss seems driven by ever larger beachfront developments, for the sake of tourism, and preferred residential locations for newly affluent people.

It should not surprise us that many kinds of animals, who used to rely on those beaches, are being severely pressured, such as certain shore birds, and sea turtles.