I didn't read all the replies, so someone may have already suggested this.
I was home alone one night, in bed, just falling asleep. Someone knocked on the door and I didn't answer. Well, they went back downstairs (I lived in a 2nd floor apt.) and climbed up onto the balcony and tried to break in the sliding glass door.
I can only guess they were knocking to see if someone was home. When the noise finally did get me out of bed, and they saw and heard that someone was home, they ran off.
I don't mean to scare you with this story, but it may be something you want to consider for your own safety.