Some strangers can knock on your house and stay there for a long time to make sure if you aren't home. If its confirmed, they will find a way to break in your house :|
Happened to me a few days ago. This guy kept ringing my doorbell and stood there for a long period of time, I pretended as if I wasn't home. After about an hour, he returned, and went to my backyard. When he saw me through a window, he got nervous and walked away as fast as he could :|