Riddle 4. A solitary individual
In the suburbs of a big city lives an elderly man who never goes far away from home. On a summer Friday, a mailman approaches his house and calls the man by name, but the latter does not answer. He then approaches the window and sees him lying on the ground in a pool of blood. The postman calls the police. Arriving at the scene, the inspector found next to the house two bottles of warm milk, a bottle of fresh milk, and a newspaper on Tuesday.
The next day, the police arrest the culprit. How did they identify it so quickly?