Don't open their door at all. The pandemic and rise in crime and political strife has traumatized a not insignificant number of people who show up in my office. They often opt for phone call visits, not even video and certainly not in person. These days, when they can work from home, get groceries delivered and dropped at the front door, and don't have to come face-to-face with another human being, it's easy to stay behind the four walls of your home, for better or worse.
I'm not saying that's definitely what happened here. The homeowner obviously socialized with people he knew, hence the 3 men. But this is in response to the many people who think it's impossible for the dead bodies to be missed. It really isn't. If this was someone who worked from home, didn't socialize much, and didn't leave the house often, I can see it happening. Until we know more about the circumstances, I'm not ruling out that the homeowner legitimately didn't know.
JMO