The Last Escape

In this final detective novel to feature Superintendent Robert MacDonald, we find the police officer setting up his retirement plans on a hill farm to the south of Lunesdale. Not quite ready to retire, he buys the farm and installs a young couple to oversee his property while he's away detecting. Meanwhile, one foggy morning Rory Macshane who has just finished his first year of a 10-year prison sentence at Dartmoor sees his plans for escape come to fruition.
He has hidden away bits and pieces of this and that over the past year and when the fog begins to thicken while he out on a work-gang he takes advantage of it and disappears into the mist with enough gear to help him truly escape. About a month after the prison break, MacDonald accompanies the farmer who has been renting the adjoining land on an tour of the abandoned farm house. There they find that someone is lying dead in the house.
Is it murder or an accident?