A lot of people have pet doors in their back door.  You know what they are:  small swinging doors or plastic covers etc. that are unlocked so your cat or dog can get out without a human having to open & close the door.  But if you cat or dog can get out, a person can get in.  You might be surprised to discover how big the pet door is (or how small a person can make her/him self).  Anyway, pet doors leave your home unsecured.  So when you are locking your doors you should do something about the pet door too.