Tinker, Tailor, Soldier, Spy (The Karla Trilogy, #1)

by John Le Carré

There were two locks, a Banham deadlock and a Chubb Pipekey, and two splinters of his own manufacture, splits of oak each the size of a thumbnail, wedged into the lintel above and below the Banham. They were a hangover from his days in the field.