Thanks to Kmarg for the following.
Season 4 Final, Juliet and Sawyer sitting on the beach, staring at the column of smoke where the freighter exploded.
When Juliet and Sawyer were looking towards the column of smoke the helicopter was on it´s way back to the island. As they were short on gaz, Frank took the shortest way back hoping to reach the island before they´re out of fuel. He headed straigt towards the beach where Juliet and Sawyer where sitting. Only a few minutes after that te sky turned purple and the island moved. Sawyer and Juliet must have seen the helicopter coming back. It was not too far away, because in a scene before when Daniel was picking up the second group of losties we could see the freighter from the beach. Even if the helicopter headed towards the island in a different angle, they couldn´t have missed it.