I recently read a book "117 days adrift" and these people were passed by seven ships within a mile of there position, they had flares and made smoke bombs, and all seven passed by without notice.
It's shocking how tough it is to spot even large objects on an open ocean. During an Atlantic crossing a few years ago, we would hail every ship we came across and at least 50 percent said they couldn't see our 34-foot sailboat even when we gave them a bearing, distance and exact location. We were nearly run down twice, once by a cargo ship and another time during some stormy weather by another small sailboat flying a spinnaker is 40 knots of wind and 15-foot seas.
It's easy to imagine someone on a small liferaft, or floating alone in the water, being overlooked.
