It bloody hard spotting things from the air. Even if you now exactly where to look. I remember doing some work for the BBC in Africa for Mr Ray Mears. We were looking for the crew on the ground in a Landie and we were in radio contact. They could see us but none of the three on board could spot them. I got them to drive so that they would kick up a dust cloud, then it was easy.
Also know a guy who was down in the water with three others in yellow life jackets and the search plane flew right overy them at low alt but failed to spot them because they were below the aircraft.
Now I am not a SAR pilot but I can say I have been involved in a couple of searches, and it aint as easy as you think to spot things from up there......in the sky.