In the epilogue Sir Thomas Legge, Assistant Commissioner at Scotland Yard asks Inspector Maine why the people of the town didn't realize something was wrong. The Inspector tells him that a man by the name of Morris made all the arrangements and bought Soldier Island for a man named Owen.
"And it was he (Morris) who explained to the people down there that there was some experiment on --some bet about living on a 'desert island' for a week --- and that no notice was to be taken of any appeal for help from out there." (pg 273)
Maine also told Legge that Fred Narracott, the man who took the group out to the island told him he was surprised to see what sort of people they were.
"Not at all like Mr. Robson's parties. I think it was the fact that they were all so normal and so quiet that made him override Morris's orders and take out a boat to the island after he'd heard about the SOS signals." (pg 274)
And then the weather was bad, so it was difficult to get to the island.
"No fishing boats out. There's a storm coming. Damned nuisance you can't see the village from here. We could signal or do something." (pg 125)
The SOS signals were seen on the 11th, but they couldn't get to the island until the 12th.
"There was a big sea on after the storm." (pg 274)