N or M? - Agatha Christie (Tommy & Tuppence Mysteries #3)

The man circled slowly round the house. Tidy little place he thought. Quiet, peaceful. Then a long-drawn grunt came to him.

Hallo, so they kept pigs did they? Funny— seemed as though it was underground. An odd place to keep pigs. No, it was someone having a bit of shut-eye. But who’d want to kip in a cellar?

He moved right up to a wall of the house. There, that’s where it was coming from— through that grating. Grunt, grunt, grunt. Snore, snore, snore—now that reminded him of something. ‘Cor!’ said the man, ‘that’s what it is—S.O.S!’ Kneeling down, he began to tap out a message on the iron grill of the little cellar window.