Obviously the midnight visitor was someone whom the dog knew well

"Is there any point to which you would wish to draw my attention?"
"To the curious incident of the dog in the night-time."
"The dog did nothing in the night-time."
"That was the curious incident," remarked Sherlock Holmes

Taken from Memoirs of Sherlock Holmes, Silver Blaze by Sir Arthur Conan Doyle
Didn't realise that book title was taken from a quote, but I can see why he chose it because it was an awesome piece of writing to come across. Sherlock holmes omnibus my mum bought me when I was 15 is probably one of the best things I own.