Why would geologists not have discovered Gilligan’s island?

If I remembered correctly, Gilligan’s island had an active volcano. That means even if the island was uncharted the times in which the volcano erupted and spewed chimneys of ash into the sky, that would be seen by passing ships, or aircraft that would have lead geologists to the island to investigate. I believe there were even satellites that would have caught such eruptions. How could those eruptions, however small, go unnoticed by the world, leaving the castaways stranded for all those years?