I have always enjoyed the unexpected visitor.
Many years back when my children were small, we actually lost the key to the back kitchen door, it remained unlocked, known to close friends and family. I would sometimes arrive home from shopping, etc. to find a friend or two in my kitchen, drinking a cup of coffee waiting for me. I loved that.
Obviously, grew more security minded as the years past. But was nearly always happy for the unexpected ring at the front door and a friend saying they were passing, etc. If I really was too busy at that moment, or had an appointment to get to I would tell them.
Nobody seems to do that now. Will usually, at least telephone before they come, except some of my g.children living nearby who will still occasionally, just turn up.