It's confusing that initially it was reported that her daughter Annie drove her home and was the last person to see her but, later reports said it was her son-in-law (Tomasso). There were also reports yesterday that the police were at the son-in-law's school (he is a teacher).
Of course, we only know what is in the press and sometimes it is inaccurate and, as the last people to see Mrs Guthrie, it would make sense for the police to thoroughly check them out so, the perpetrator may still have been person(s) unknown to the family.
Having said that, whoever it was went to a lot of trouble (and risk) to hang around and disable cameras etc. Personally, I think it would not be surprising if the ransom notes turn out to be a distraction. Whatever really happened that night, I hope Mrs Guthrie is found alive and returned safely to her family.
@CallMeKate The media here described it as a septic tank.
The SIL is certainly getting a lot of attention ….