The Pemberton Legacy - 1, #2

Sue Baylock has discovered a very special secret about the house that she and husband Alan bought when they decided to downsize. However, it would seem that somebody else is also interested in their house - but why? Has it got something to do with the secret and why has Sue's married daughter suddenly disappeared with a total stranger, apparently of her own free will? Events take a sinister turn when Alan discovers a stranger watching the house in the dead of night.
Who is he and why is he watching the house?