When someone rings your doorbell randomly and you aren't home, they may be casing your home.

The same thing goes for people knocking on your door or someone you don't know is walking up to your house.<br><br>I've heard of this before and know two people who have had it happen to them. Pretty much someone will ring the doorbell to see if anyone is home, and if no one is, they'll break in.<br><br>A good way to prevent this is to put some stickers of security systems on your door. Even if you don't have an alarm system, they won't know that.<br><br>EDIT: Oh my wow... didn't expect this to blow up. Thanks for the awards!<br><br>EDIT 2: I forgot to mention that leaving a tv or radio on also helps.