The average person, read criminal, has available to him the Locksmith tool of the trade. They can use a "bump key" and open your lock leaving no sign of forced entry.
I heard about this last night on a radio news program and after doing a Google and You Tube search, I am amazed at the amount of information out there. There are tons of instructional videos to teach you how to make your own bump key and use it successfully.
This is a brief news clip which demonstrates bump keys and gives information on protecting yourself against this happening to you.