Someone was at the door, probably someone heard the man screaming.
When the cops came after getting a complaint from a scream the neighbors heard, I was totally calm and wasn't nervous like most people would be, because I knew they wouldn't find Anything.
After the police officers found nothing, they were calmly talking when I started to hear the old man's heart beat. It got louder and louder 'til I couldn't take it anymore. Then eventually I ended by confessing that I had killed the old man.