After seeing the finale, it's obvious that Martin Bellamy is supposed to be the Thompsons' son, Robert.
The Thompsons, going by their clothing and the discussion about the flood in which they all died, had to have drowned in the 50's, but Bellamy would have been born in the late 60's to early 70's - I'm not sure if they've ever stated his age. So he's the first "return."
He returned 15-20 years after his death, turned up who knows where, and as an infant or young toddler couldn't provide the information required to tell the people who found him who he was. To the authorities, he would have been simply an abandoned child, healthy but with no family. He was put into the system, given a new name, and never knew that he'd lived another life.
That would have made for a nice mid-season reveal, but it falls flat at the end of the show. If they wanted to make an impact with it, then they should have let the audience in on it earlier, and had him killed during the show, only to return like Rachel did.