I think it could be either Mr. Hastings or Melissa because they were supposedly in Philly to go get Spencer's car and maybe both of them dug her in the backyard. But the main question is why their own backyard? That's what I am confused about.
It could MAYBE be Jenna because she might have come back but she is hiding somewhere.
Now I think it's Mrs. Hastings because she went to the spa for a couple of days and Spencer was confused. Mr. Hastings tried to hide something for sure.