I am wondering if it could have been an old note Rachel had written, but I'm also wondering if it could have been an old note that someone else had written, with the last line and Rachel's name added in.
So, with that first thought, maybe some time previously, Rachel wanted to take off to Houston for a bit. It could have even been prior to her marriage when she still lived with her parents. Maybe Rachel and her sister were planning on running away for whatever reason, and that note was written and meant for their parents, but then they didn't end up going through with it... So nobody ever saw the note, yet it still existed, maybe hidden in a folder or notebook or what have you, and then someone who knew about it decided to use it to make it seem like the trio ran off. MOO