Gabby broke into Emma’s house. If Gabby found the letter, she may have given it a quick glance and got scared that Emma had written about her in the letter? And by the time she realised that maybe she hadn’t put anything about what Gabby had done in there, it was all round the village that Emma had attacked Moira, stolen a baby and killed her own son, so maybe she felt it was too late to hand the letter in?
Somebody’s definitely taken it, because Emma left it on the baby’s lap and when Harriet turned up and found the baby in the church you could see that the letter had disappeared as Harriet bent down towards the baby. So somebody has it unless the wind blew it away....