This afternoon, I found myself writing to one of my ESL students:
I am imagining you and your dog having a fine time at the cabin as I write this.
I bet you are in the cabin as well. In the first sentence at the cabin is correct, just as you would say "I am at home" rather than in home. It would also be correct to say "I'm in the house" rather than outside in the yard.
When you are at home, the yard is included. When you are in the house, the yard is excluded. With cabin, the same word is used both ways. When you are at the cabin, the exterior property...