One time someone knocked on my door. I said "Who's there?". Nobody answered. I repeated myself, and nobody answered again.
I wanted to find out who it was but realized that if I opened the door, I would be at a real disadvantage. One hand would have to be on the knob, giving me only my left hand to defend myself. Also, when opening a door, you stand right in the opening, unless you try to hid behind it.
So After asking who it was twice without getting an answer, I waited about 20-30 seconds, unlocked the door very quietly, and backed off. I yelled something like "I'm not coming out, why don't you try coming in?" and "The door is unlocked!" I was far enough away from the door, I didn't have to worry about hand to hand combat. I was also off to the side, out of sight, just in case whoever it was had a gun. (The gun would have been pointed straight forward).
Anyway, I felt able to handle the situation, and whoever it was must have realized this. I waited a little while, then checked the area, whoever it was left. For some odd reason they did not accept my warm invitation to come on in.