He is a black Labrador retriever, 3 years old and neutered. We acquired him at 8 months old, with some behaviour problems. Happily, with some training (Kennel Club bronze, silver and gold awards), plenty of exercise and mental stimulation, he has calmed down and is now a well behaved, calm, friendly dog.
When the curtains are closed he seems to lie down and rest more easily, as he can't see people and dogs, horses, bikes, whatever, going past the end of the garden. However, if anyone going past makes any noise he is straight up at the window and barking madly. We had people here yesterday doing work in the garden and he was ok once he had been introduced to them - he then sat in the window and just watched what was going on. I wonder if his reaction to people passing by is a "guarding" response.....?