Are you for real? Because the front door is unlocked, you're assuming that the house owner is happy for people to open the door and let themselves in? What planet do you live on?
What if the house owner simply forgot to lock the door?
What if (like me) the house owner is taking various bits of rubbish out and doesn't want to lock the door between trips?
What if the house owner is waiting for their girlfriend to come around and is waiting naked in the bedroom for them?
The only time it is acceptable for a delivery man to open my unlocked front door and drop a parcel off is if I have planned it with them previously. Other than that, they can knock, wait for me to answer, and card me if I'm not there. If I come down the stairs and find that delivery man in my house, I wouldn't ask him what he was doing, I'd be treating him as a burglar and tackling him to the floor.