Dammit!!! Still nobody home. 
Okay, so they've had more than enough time to get home from shopping/going out for a meal/to catch a movie.
So, one could conclude:
1. They're on vacation
2. They're in Jail
3. They're in Hospital
4. They've contracted an ENT infection and can't hear the doorbell
5. They've discovered Viagra and are pretending to be out
6. They were murdered in their beds and are unable to answer the door.
So, if anyone calling in the next day or so notices an awful smell. it may be the latter and will need to call the cops.
There again, if there's a funny smell as a result of #6, call the paramedics because will have been longer than 4 hours, and neither can reach the phone.
